In the tenth season premiere, Penny’s family arrives for the wedding ceremony, including her anxiety-ridden mother, Susan (guest star KATEY SAGAL), her drug-dealing brother, Randall (guest star JACK McBRAYER) and her father Wyatt (returning guest star KEITH CARRADINE). After Sheldon’s mother, Mary (recurring guest star LAURIE METCALF) and Leonard’s father, Alfred (returning guest star JUDD HIRSCH) share an evening together, however, everyone deals with an awkward morning the next day. Recurring guest star CHRISTINE BARANSKI returns as Leonard’s mother, Dr. Beverly Hofstadter, while Breaking Bad’s DEAN NORRIS guest stars as Air Force Representative Colonel Williams.
Tensions rise when the guys struggle to complete their government project on time – and Sheldon tries an energy drink to stay awake. Also, Penny and Amy go to a “party” at the house of Bert, the geologist (guest star BRIAN POSEHN). And Raj learns Bernadette’s true feelings about her pregnancy when they clean out the future baby room. Dean Norris returns as Air Force Colonel Williams.
Leonard and Penny must separate a quarreling Sheldon and Amy, when their cohabitation experiment doesn’t go as planned. Sheldon threatens to break off their relationship when Amy refuses to adhere to the “bathroom schedule.” Also, Howard and Bernadette find unexpected guests at their house when they decide to stay home from a planned vacation.
Penny is surprised to learn she has fans of her performance in “Serial Ape-ist” when she attends the Van Nuys Comic-Con with Leonard. Also, Amy hosts a Sheldon-style brunch at Penny’s apartment. And Howard makes an impulsive purchase when he starts stressing out about the baby on the way.
In order to continue living with Sheldon, Amy lies to him about repairs being made to her apartment; and they host an “on location” episode of “Fun with Flags” in Penny’s apartment. Also, Leonard lets Penny decorate Sheldon’s old room after she learns that she has secretly been moving Leonard’s collectibles into storage.
After a successful experiment combining their genes, Sheldon will stop at nothing to convince Amy they should procreate. Also, Raj is embarrassed to tell the gang what the new woman he’s dating does for a living.
When CalTech geologist Bert (recurring guest star BRIAN POSEHN) wins the MacArthur Genius Fellowship, Sheldon is overcome with jealousy. Meanwhile, Howard finds an old remote control Stephen Hawking action figure he invented. STEPHEN HAWKING returns to guest star as himself.
A battle ensues when Leonard and Sheldon argue over who will get custody of their shared belongings. Also, Raj and Stuart fight to be the most helpful during Bernadette’s final weeks of pregnancy. CHRISTOPHER LLOYD guest stars as Theodore.
After Penny feels that Leonard is taking her for granted, she decides to go on a spa weekend with Amy, leaving Sheldon to play the unlikely peacemaker. Meanwhile, Howard and Raj try to fix a squeaky floorboard in the baby’s room.
Sheldon beta tests an MIT invention that helps him read people’s emotions. Meanwhile, Raj invites his ex-girlfriends over to give him dating advice, and Leonard can’t hide his feelings when Penny announces that her brother is coming to town. Guest stars LAURA SPENCER, KATE MICUCCI, ALESSANDRA TORRESANI and KATIE LeCLERC return as Raj’s ex-girlfriends.
Leonard and Howard try to distract Sheldon with tickets to a historic railway when he slows the progress of their guidance system. Meanwhile, Penny and Amy take Bernadette out for the night, leaving Raj and Stuart to care for baby Halley. Dean Norris returns as Colonel Williams.
Sheldon and Amy have their first fight since moving in together, after he shares intimate details of their relationship with people at the university. Meanwhile, Raj is shocked by the realities of financial independence when he stops receiving support from his father (guest star BRIAN GEORGE).
Sheldon pushes himself to the limit when he collaborates on projects with Leonard and Howard as well as Amy on the same day. Also, Bernadette nears the end of her maternity leave and must return to work. JOEL MURRAY guest stars as Doug.
After Leonard, Sheldon and Howard celebrate the completion of their top secret Air Force project, they’re met with an unpleasant surprise. Also, Amy is offered a summer position as a visiting researcher.
In the ninth season premiere, Penny struggles with Leonard’s confession that he kissed another girl while on the North Sea. After driving to Vegas, Penny must now decide whether she wants to go through with the wedding. Back home, Sheldon doesn’t know how to act after Amy pushes pause on their relationship.
After Amy breaks up with Sheldon, he tries to retaliate by filming a special episode of “Fun with Flags.” Leonard, meanwhile, confronts the woman he kissed on the North Sea in an attempt to put Penny’s mind at ease, and Bernadette feels guilty about withholding a secret from Penny.
The guys’ science skills are put to the test when they get a flat tire during Leonard’s surprise bachelor party weekend in Mexico. Meanwhile, the girls force Penny to finally tell her family that she eloped with Leonard.
When Penny and Leonard announce they’re moving in together, Sheldon decides to revert back to 2003 – a simpler time before he met either of them. Meanwhile, Howard and Raj form a band to play at the Comic Book Store.
When the guys take fencing lessons from Barry Kripke (recurring guest star JOHN ROSS BOWIE), Sheldon finds himself in an awkward position when Kripke expresses interest in Amy. Also, Stuart struggles to bring in female customers into the Comic Book Store, so Penny, Amy and Bernadette come by to help him out.
In the midst of a nationwide helium shortage, Sheldon and Leonard take desperate measures to obtain the supplies they need on the black market. Meanwhile, Penny and Bernadette download a dating app on Amy’s phone in order to find her a new man. MICHAEL RAPAPORT guest stars as their Black Market dealer, Kenny.
While being interviewed for a documentary about Spock, Sheldon struggles to suppress his emotions about his recent break-up with Amy – leaving Leonard and Penny shocked to discover that Sheldon had an engagement ring for Amy. Meanwhile, Howard and Bernadette disagree over remodeling his childhood home. WIL WHEATON, ADAM NIMOY and STEPHEN MERCHANT guest star.
With Amy out on a date with a mystery man, Sheldon enlists the help of Howard and Raj to find a new girlfriend by posting online. And when Amy is coy with the details about the guy she’s dating, Penny and Bernadette decide to spy.
Sheldon and Amy try hanging out as friends when they spend Thanksgiving together at the Aquarium. Meanwhile, Howard reluctantly agrees to volunteer at the soup kitchen with Bernadette, Raj and Emily, while Penny discovers that Leonard knows more about her than she thought.
After driving Leonard and Penny crazy by his desperate attempts to remember the name of a song stuck in his head, Sheldon has a revelation when he realizes why. Then, despite their previously awkward date, Amy invites Dave (recurring guest star STEPHEN MERCHANT) over for dinner. And Howard and Raj become obsessed with a fan of their band, Footprints on the Moon.
After more than five years of dating, Sheldon and Amy spend their first night together; while Leonard, Howard and Raj must decide who will take their extra Star Wars movie ticket. Recurring guest star BOB NEWHART returns as Sheldon’s Obi-Wan to advise him on his difficult relationship decisions.
Sheldon’s thrilled when his Meemaw (guest star JUNE SQUIBB) comes to visit, but his excitement quickly fades when she butts heads with Amy. Also, Raj meets a new girl (guest star ALESSANDRA TORRESANI) at the comic book store who makes him question his relationship with Emily.
Sheldon and Amy host a special live Valentine's Day episode of "Fun with Flags." While Leonard and Penny grapple with getting older, Howard and Bernadette find a surprise in their hot tub; and Raj is torn between Emily and the cute girl he met at the Comic Book Store.
When the gang finds out about Bernadette’s pregnancy, they celebrate by singing karaoke. Meanwhile, Bernadette is concerned when Howard freaks out over her pregnancy announcement, as he’s convinced that he needs to make more money to provide for the baby.
After more than nine years together, the gang finally celebrates Sheldon’s birthday with a few special guests on the 200th episode. CHRISTINE BARANSKI, SARA GILBERT and WIL WHEATON return and are joined by special guest star ADAM WEST.
Leonard, Sheldon and Howard run into problems when they file for a patent for their infinite persistence gyroscope. Meanwhile, Penny, Amy and Bernadette give Raj dating advice after Emily reaches out to him after their breakup.
When Sheldon’s laptop dies, it leads him to share a deep secret with Amy after she buys him a new one; Meanwhile, Raj sells out Leonard and Howard after they lie to their wives in order to attend an early screening of a movie.
The gang goes to a wine bar and runs into Penny’s old boyfriend, Zack (recurring guest star BRIAN THOMAS SMITH); Bernadette gets stuck spending the evening with Sheldon when she can’t go wine tasting with the group, and things don’t go as planned when Raj introduces his new girlfriend, Claire (ALESSANDRA TORRESANI).
Chaos ensues in the ninth season finale when Leonard’s recently divorced parents (special guest stars JUDD HIRSCH and CHRISTINE BARANSKI) and Sheldon’s devoutly religious mother come to town. Meanwhile, Howard and Raj think the government is out to get them, when they’re contacted about their guidance system.
Leonard and Sheldon (series stars JOHNNY GALECKI and JIM PARSONS) are brilliant physicists – geniuses in the laboratory, but socially challenged everywhere else. Enter beautiful, street-smart neighbor, Penny (series star KALEY CUOCO-SWEETING), who aims to teach them a thing or two about life. Despite their on-again, off-again relationship in the past, Leonard and Penny have finally gotten engaged. Even Sheldon has found a female companion – entering into a “Relationship Agreement” with neurobiologist Amy Farrah Fowler (series star MAYIM BIALIK). In their free time, Leonard and Sheldon enjoy fantasy role-playing games with their ever-expanding universe of friends, including fellow scientists Raj (series star KUNAL NAYYAR) and Howard (series star SIMON HELBERG), and Howard's adorable microbiologist wife, Bernadette (series star MELISSA RAUCH).
When Sheldon is forced to teach a class, Howard surprises everyone by taking it. Meanwhile, the tension between Penny and Bernadette gives Amy a chance to play both sides. Guest star REGINA KING returns as Mrs. Davis, the University’s Director of Employee Relations.
Howard is nervous when NASA asks him to throw out the first pitch at a Los Angeles Angels game for National Space Day. Also, Leonard and Penny try to prove Sheldon wrong after he claims that he and Amy are a superior couple.
Amy and Bernadette accuse Penny of being a “buzzkill” after she gets an email from work during their Vegas weekend. Meanwhile, the guys try to invent something cool, but only come up with new ways to procrastinate.
Sheldon and Raj test whether they could survive a dark-matter research expedition in a salt mine by simulating the conditions in a steam tunnel deep underground. Meanwhile, Leonard and Penny seek advice from Wolowitz and Bernadette after they fight about money.
The guys try to help Penny out of an awkward work dilemma, when a smitten doctor shows up at her door after her flirtatious sales techniques work a little too well. Meanwhile, Amy and Bernadette argue over whether or not female scientists should play up their sexuality.
While Sheldon says a tearful goodbye to the final installment of “Fun with Flags,” Leonard, Howard and Raj search for something significant in a dead professor’s research. And Bernadette is shocked to learn of her co-workers’ true feelings from Penny.
Leonard and Howard turn to Raj for help in dealing with a work emergency when they nearly come to blows after blaming each other for a potential disaster at the university. Amy throws an authentic Victorian Christmas party, while Sheldon seeks revenge on her for making him celebrate the holidays
After Bernadette finds Penny's gorilla movie online, Penny manages to find an even more embarrassing video about Bernadette. Leonard and Sheldon upload a paper to the Internet and obsess over someone making negative comments about it.
Howard receives some shocking news as everyone prepares for the reopening of Stuart’s comic book store. Penny teaches Sheldon how to “let it go” but can’t follow her own advice after she learns something infuriating about Amy. Also, Leonard and Raj think they spot Nathan Fillion at a restaurant.
When the gang hears about an experiment designed to make participants fall in love, Sheldon and Penny put it to the test as a joke. Meanwhile, Leonard, Amy, Raj and Emily spend the evening trying to escape a room with a “zombie.” And Bernadette and Howard run into trouble at the airport after returning from the funeral for Mrs. Wolowitz.
A trip to the pet store ends in a fight when Amy learns that Sheldon has applied for a one-way mission to colonize Mars. Also, Leonard surprises Penny with a very “adult” purchase; and Raj is caught snooping in Emily’s apartment.
When Leonard and Sheldon are invited to speak at UC Berkeley, they take a detour to try to sneak on to George Lucas’s Skywalker Ranch to meet one of their idols. Back in Pasadena, Howard and Bernadette battle over the fate of his Dr. Who TARDIS, while Raj and Amy offer to settle the dispute with a game of ping pong.
When Sheldon and Amy build a blanket fort, they come to a big crossroads in their relationship. And Wolowitz is shocked when a man claiming to be his half-brother he never knew he had shows up at his front door. And, when Penny is invited to appear on Wil Wheaton’s podcast, she and Leonard have a huge, on-air argument.
When Raj is asked to create a message in case a NASA mission discovers alien life, the guys fight over what he should do. Meanwhile, Penny is torn between auditioning for a movie or keeping her successful pharmaceutical sales job.
Wolowitz questions his engineering abilities when he and Sheldon can’t get a drone to fly. Meanwhile, a canceled flight nearly prevents Leonard from giving the commencement address at his former high school. And Raj pits his parents against each other when his father cuts him off financially
Personalities collide and sparks fly when Sheldon and Leonard’s mothers (special guest stars LAURIE METCALF and CHRISTINE BARANSKI) finally meet. Meanwhile, Howard decides to “man up” and do his fair share of the housework – but not without a little help from his friends.
On the one-hour seventh season premiere, Sheldon and Penny bond in Leonard's absence, but it's Sheldon's feelings that are crushed when Leonard returns from the North Sea. Guest star REGINA KING returns as Mrs. Davis, the University's Director of Employee Relations.
On the second part of the one-hour seventh season premiere, Raj gets consoled about his ex-girlfriend and Howard's relationship with his mother causes an unusual threat to his masculinity. KEVIN SUSSMAN returns as Stuart, the comic book store owner.
After Amy ruins one of Sheldon's favorite movies -- Raiders of the Lost Ark -- he gets defensive, seeking revenge. Meanwhile, Leonard discovers a way to get Penny to do anything; and Raj and Stuart create online dating profiles.
Sheldon must decide how much "Amy time" is too much after she takes a job at his university, and their relationship is put to the test. Meanwhile, Howard finds himself sleeping on Raj's couch after a fight with Bernadette.
Sheldon's latest scientific breakthrough makes him feel like a fraud -- and threatens to haunt him forever. Meanwhile, Howard's romantic gesture to Bernadette causes Penny to step up her game with Leonard.
Sheldon feels slighted when Professor Proton (recurring guest star BOB NEWHART) seeks advice from Leonard instead of him, and he seeks revenge by befriending a rival science TV host, BILL NYE (guest starring as Himself). Meanwhile, Raj gets jealous when Howard crashes "girls' night."
Leonard tries to keep Sheldon from overreacting when a past mistake comes to light, but Sheldon punishes Leonard by making him "walk a mile in his shoes." Meanwhile, Penny confronts Raj's ex-girlfriend, Lucy (guest star KATE MICUCCI).
Leonard and Amy help Sheldon cope with the shame he's feeling after his accidental contribution to science is disproved. Meanwhile, Raj must stay with Howard and Bernadette for a week. IRA FLATOW guest stars as Himself.
When Penny's big acting break on NCIS is a bust, Leonard struggles to help her, which results in Penny asking Leonard a bold relationship question. Meanwhile, Sheldon tries to learn how to be funny and Raj tries to work on his "game" before talking to girls.
When Sheldon is forced to take a vacation, he tries to relax, but instead spends a lot of time with Penny after she quits her job. Meanwhile, Leonard struggles to be supportive of Penny and Bernadette seeks Stuart's help in replacing one of Howard's comic books.
After they guys can't get Comic-Con tickets, Sheldon decides to hold his own convention, and winds up spending a wild night with JAMES EARL JONES (guest starring as himself). Meanwhile, the girls see if they can act like "grown-ups." Star Wars icon CARRIE FISHER makes a cameo appearance as well.
Love is in the air when Amy convinces Sheldon to join her (as well as Howard and Bernadette) for a romantic weekend in Napa Valley to celebrate Valentine's Day. Meanwhile, Leonard and Penny must rush Raj's dog to the vet.
When Leonard buys a dining room table, it causes Sheldon to reevaluate the changes in his life. Meanwhile, Wolowitz is offered a chance to go back to space and Bernadette struggles with whether or not to encourage him.
While Raj is wracked with guilt over dating two women at the same time and Penny is wrestling with whether to take a role in a cheesy movie, Amy must help Sheldon through the even greater hell of choosing between two gaming systems.
Leonard tries to help Sheldon free his mind by reinstating "Anything Can Happen Thursday," but when Sheldon tries to be spontaneous, it leads to unexpected friction between Penny, Amy and Bernadette. Meanwhile, Raj seeks Howard's help in preparing for a date with Emily.
May the "Fourth" Be With You! Professor Proton helps Sheldon cope with grief, while Leonard turns a relationship milestone into a competition with Penny. Meanwhile Amy and Bernadette make a "Star Wars Day gift for the guys.
A horrible day at work prompts Penny to evaluate her life choices -- including taking a big step with Leonard. Meanwhile, Howard and Bernadette struggle to care for Mrs. Wolowitz, and Raj gets relationship advice from Sheldon.
With his entire world changing around him, Sheldon is forced to confront an uncertain future and considers a major move in the seventh season finale. Meanwhile, Raj and Emily take it to the next level, and Mrs. Wolowitz's injury is a major headache for Howard and Bernadette. Recurring guest star CHRISTINE BARANSKI returns as Leonard's mother, Beverly Hofstadter.
On the sixth season premiere, Howard discovers that he can't escape his mother -- even in space! Caught in an argument between Bernadette and his mom (recurring guest voice CAROL ANN SUSI), Howard deals with woman trouble from the International Space Station. Back on earth, meanwhile, Raj faces his loneliness, by turning his existence as a third wheel into an art form.
When Sheldon learns that Penny is thinking about breaking up with Leonard, he tries to intervene. But Sheldon is no good at keeping secrets. Meanwhile, Howard is being picked on in space by the other astronauts (guest stars PASHA LYCHNIKOFF and MIKE MASSIMINO).
The gang celebrates Halloween at Stuart's comic book store. Meanwhile Leonard and Penny discover a new spark in their relationship when he seduces her with science. And while Howard can't stop talking about space, it takes iconic astronaut BUZZ ALDRIN (guest starring as himself) to get him grounded again.
Pressured by Bernadette, Howard comes to terms with moving out of his mother's house. Meanwhile, Sheldon, who's caught in the middle of a feud between his "girlfriend" Amy and good friend WIL WHEATON, goes to Penny for advice. Star Trek's LeVAR BURTON guest stars.
Sheldon unwillingly revisits some Christmas memories during a lively game of Dungeons and Dragons, while Penny, Bernadette and Amy try to find the perfect girl for Raj when he joins the girls for ladies night.
Sheldon is accused of sexual harassment at the Univesity and winds up dragging Leonard, Raj and Howard into the fray. Southland's REGINA KING guest stars as the University's Director of Employee Relations.
While the guys take a road trip to a comic book convention in Bakersfield dressed as characters from Star Trek: The Next Generation, Leonard's car is stolen. Meanwhile, the girls stay home and try to appreciate the world of comics.
Outraged that he's being forced to work with his university nemesis, Barry Kripke (guest star JOHN ROSS BOWIE), Sheldon faces a crisis of conscience. Meanwhile, Howard and Raj spend a thousand dollars on action figures of themselves.
A fight between Leonard and Sheldon drives Leonard to finally move in with Penny -- not realizing how the shift will affect Penny and Amy's living arrangements. Meanwhile, Raj takes care of Mrs. Wolowitz while Howard is away.
Back to school! Leonard, Sheldon and Howard try to advance the cause of women in science by speaking with junior high girls about their future careers, while Raj plans his date with Lucy (guest star KATE MICUCCI).
When Howard finds a letter from his father, he's forced to confront his past, struggling with whether or not he should open it. And Leonard and Penny throw a "grown-up" cocktail party at the apartment.
Sheldon feels lost after one of his favorite TV shows gets canceled too soon, so Amy tries to help him get over his compulsive need for closure. Meanwhile, Raj discovers a secret about his new love interest, Lucy.
When the girls' trip to Vegas falls through, the guys invite them to play Dungeons & Dragons -- causing Sheldon and Amy's relationship to take an unexpected turn. Meanwhile, Raj and Lucy go on a very awkward date.
When Leonard is offered an exciting new job opportunity overseas, Sheldon grows very jealous and Penny is thrown for a loop. Meanwhile, Raj pushes Lucy a little too far in their relationship on the sixth season finale.
The fifth season premiere picks up where the fourth season finale left off as the gang deals with the aftermath of Penny and Raj's night together. While Penny worries that she's screwed things up permanently with her friends, Sheldon takes command of the paintball team. Emmy® nominee CHRISTINE BARANSKI (The Good Wife) reprises her role as Leonard's emotionally distant mother.
A battle of wills between Sheldon and Penny leaves Amy caught in the middle, while Leonard tries to spice up his long-distance relationship with Priya (recurring guest star AARTI MANN), who has moved back to India.
Leonard attends a co-worker's wedding as Amy's date. Meanwhile, Howard tries to convince Bernadette to live with his mom after they get married by suggesting a trial run with the two of them staying with his mom over the weekend.
When Raj is upset he's the only one of the four guys who is not in a relationship, Penny sets out to find the perfect date, introducing him to Emily (guest star KATIE LeCLERC -- Switched at Birth) a girl he can actually talk to!
Sheldon's old nemesis, WIL WHEATON (guest starring as Himself), returns and invites everyone to a housewarming party. Sheldon threatens to "defriend" Leonard if he attends. But when he hears that BRENT SPINER (guest starring as Himself), the actor who played Commander Data, will be there, Sheldon decides to go to the party after all... only to discover new levels of treachery. Meanwhile, Howard gets an out-of-this-world opportunity that Bernadette reacts to strongly.
Sheldon gets in trouble with Amy when he isn't impressed by her recent accomplishment, and it's up to Penny to teach him how to be a better boyfriend. Howard, meanwhile, must come to grips with Bernadette's dislike of children.
The possibility of Leonard and Penny getting back together could prove disastrous when Leonard offers Penny a spur of the moment invitation to a romantic dinner for two on the 100th episode of the series.
Sheldon's selfish demands force Leonard to reconsider their friendship. Could this be the end of The Roommate Agreement? Meanwhile, Howard tries to pick his astronaut nickname, but NASA astronaut MIKE MASSIMINO (guest starring as himself) has other ideas.
Sheldon's life is turned upside-down after his barber becomes hospitalized. Now, that there's been a change in his routine, Sheldon finds himself re-evaluating his entire life. Meanwhile, astronaut training causes Howard to rethink his decision to go into space.
It's on...like Alderaan, when the guys plan a weekend Star Wars gaming marathon! But when Sheldon chooses the guys-only weekend over a weekend with Amy to visit her aunt, he is destined to feel the force!
It's a dream come true for Sheldon when he has a close encounter with his hero, Mr. Spock. Legendary sci-fi icon LEONARD NIMOY guest stars, lending his voice to a unique reprisal of his three-time Emmy®-nominated Star Trek role.
When NASA reschedules Howard's mission and puts his wedding plans in jeopardy, he's forced to face his fears and Bernadette's dad (guest star CASEY SANDER). Meanwhile, Leonard says something surprising to Penny in the bedroom.
Howard Wolowitz crosses over into the final frontiers of space...and marriage on the fifth season finale. When Howard and Bernadette decide they want to be married before his NASA space launch, the gang rushes to put on a wedding. NASA Astronaut Mike Massimino guest stars as Himself.
On the fourth season premiere, Penny (series star KALEY CUOCO) finds herself along for the ride on Sheldon's (series star JIM PARSONS) first date ever. Meanwhile, Wolowitz (series star SIMON HELBERG) finds a new use for a robotic arm. MAYIM BIALIK guest stars as Sheldon's date, Amy Fowler.
An embarrassing secret of Wolowitz's comes to light, while Sheldon and Raj have a small war at work. Battlestar Galactica's KATEE SACKHOFF and Star Trek's GEORGE TAKEI guest star as themselves as Wolowitz's love gurus. MELISSA RAUCH reprises her role as Howard's girlfriend, Bernadette.
When the guys camp in line for a screening, their plans are thwarted by Sheldon's greatest nemesis, WIL WHEATON (guest starring as himself). Meanwhile, Penny and Bernadette invite Amy to her first slumber party.
A wealthy donor to the university makes Leonard consider how far he's willing to go for the sake of science. JESSICA WALTER (Arrested Development) guest stars as the widowed benefactress and JOSHUA MALINA (The West Wing) guest stars as the University President.
When Bernadette receives her Ph.D., the guys take joy in reminding an emasculated Wolowitz that he is the only one in their group who isn't a doctor. Meanwhile, Koothrappali becomes Sheldon's new roommate after hearing Leonard and Priya engaging in a Star Trek bedroom fantasy, on the fourth season finale.
On the third season premiere, Sheldon (series star JIM PARSONS) flees back to Texas in disgrace when he learns that the guys tampered with his arctic expedition data -- forcing his friends to follow him, and threatening Leonard's (series star JOHNNY GALECKI) hopes for a romantic encounter with Penny (series star KALEY CUOCO).
Leonard and Penny struggle to recover from an awkward first hookup, while Sheldon and Howard (series star SIMON HELBERG) stake their best comic books on a bet to determine the species of a cricket. LEWIS BLACK guest stars as an entymologist who settles the high-stakes bet.
After he's humiliated on National Public Radio, Sheldon vows to destroy Kripke (guest star JOHN ROSS BOWIE), and the guys help Sheldon exact his revenge. Meanwhile, Wolowitz tries not to destroy his new relationship with Bernadette (guest star MELISSA RAUCH). KATEE SACKHOFF (Battlestar Galactica) makes a cameo as Wolowitz's dream girl.
While Sheldon and Koothrappali attend a university mixer, Leonard is upset to discover that Penny believes in psychics. DANICA McKELLAR (The Wonder Years) guest stars as the object of Raj's attraction.
It's Valentine's Day, and Leonard can only bring one guest on a trip to see CERN's Large Hadron Collider (the world's largest and highest energy particle accelerator) in Switzerland. Will it be Sheldon or Penny?
In the third season finale, Wolowitz and Koothrappali search for Sheldon's perfect match online -- and find Sheldon's ideal woman (guest star MAYIM BIALIK). Meanwhile, Penny worries that dating Leonard has ruined her for normal guys.
Penny and Leonard compete to see who will be the first to move on after their breakup. When Leonard sees Penny with a handsome new guy (guest star TRAVIS SCHULDT Scrubs), he enters into a rebound relationship with Sheldon's nemesis, Leslie Winkle (series regular SARA GILBERT).
Rajs ego puts all of his friendships in jeopardy when he gets an inflated sense of self. Newly famous because of an article in a magazine, Raj (series regular KUNAL NAYYAR) makes a disastrous play for Leonards longtime crush, Penny.