Mary returns to WITSEC from maternity leave to find a new set of office dynamics. She must protect a witness who could take down one of the most powerful secret societies in the country.

Mary must juggle WITSEC with her new childcare duties after a hoarder witnesses a violent murder. Some say it takes a village; for a US Marshall, it takes the entire office.

When Marshall's witness escapes from the WITSEC offices and disappears on an Indian Reservation, Mary and Marshall will need the help of a Native American detective to track him down.

Mary protects a bigamist not only from his former employer, but also his two wives. In WITSEC, cheating the black market can be dangerous; but cheating on multiple wives is a death sentence.

Mary must protect a drag queen from reverting back to his old ways while Marshall gets "dragged" into an undercover sting operation.

When Mary's fugitive father turns up, his return puts Brandi and Jinx in danger. Revelations abound as Mary struggles to determine whom she can trust.

With Mary's father on the lam and out for vengeance, she finds herself under FBI surveillance. In order to clear her name, she'll have to lose her tail and track down her own father before anyone else does.

In the series finale, Mary finds herself caught between her "model" witness and an unexpected family reunion, while radical changes take place around the WITSEC office.

Mary uses a thief to catch a thief when she asks a witness to consult on a crime in order to exonerate Brandi of grand theft auto.

Mary must help a whistle-blowing witness reconnect with his son as he struggles with a severe case of paranoia. Unfortunately, when you're in WITSEC just because you're paranoid doesn't mean they're not out to get you.

Mary is forced to be a reluctant go-between in the WITSEC version of a witness Romeo and a suspect Juliet.

Mary helps a Miss Congeniality teenage pageant queen cope when she suddenly realizes that she's spent her entire life in the witness protection program -- but didn't know it.

Mary learns that the only thing she despises more than protecting an unapologetic con artist, is having to team up with him in order to take down an even bigger dirtbag.

When Mary's witness is forced out of an Amish community and into WITSEC, learning to adapt is far from her only concern. Even a fish out of water has to worry about sharks.

Mary must help a terminally ill father heal a tarnished relationship with his distant children.

Mary plays marriage counselor in order to mediate a WITSEC divorce. Not only must she reconcile a stubborn witness, but also thousands of years of Hindu tradition, in order to save the case and keep a family out of danger.

Mary gets a WITSEC two-fer, when a witness is forced to enter the program after being mistaken for his "evil" twin brother. Not only must she protect the pair from their enemies, but also from each other.

When Mary's WITSEC mandated road trip turns into an introspective odyssey, Marshall and Stan are left to deal with a rockstar witness and her overbearing mother.

Mary helps protect a guilt-ridden army veteran, while butting heads with his JAG attorney. Unfortunately for Mary, things are further complicated by the return of her ex-husband.

Mary learns all the joys of childbirth while protecting a courtesan from the gangster that impregnated her. Although Mary may be immune to her charms, can a seductress truly ever be trusted?

On the day of her sister's wedding, Mary is flown to Miami in order to protect her least favorite witness from hired mercenaries.

After being discharged from the hospital with a fresh bullet wound scar, Mary gets right back to work with Marshall. They come across what appears to be video footage of an armed robbery that one of their witnesses used to orchestrate as a mobster in Boston. The witness: Jimmy McCabe. After grilling McCabe, Mary turns her attention to Jimmy's sons who are doing their best to conceal the truth about their dad. Meanwhile, Marshall's presence in gangland is starting to hurt gang-banger Lala's business. But Marshall is looking for the name of Mary's shooter. It's only a matter of time before Lala's proximity to the cops ruins his business, and before Marshall gets in too deep.

Mary and Marshall's first case together in 2003 is revisited after an amorous couple in Baltimore witnesses the brutal killings of the boyfriend's family. Marshall and a reluctant Mary must transport Henry Doer (Josh Cooke) and Claudia Tobin (Jeannine Kaspar) across the country to the safe haven of Albuquerque. Flash-forward to 2010: Mary and Marshall are rehashing this first case when the couple - now on the road to romantic ruin - jeopardizes their identities when Henry comes to realize that the romance has died. There's also a new face at WitSec headquarters. The newly appointed U.S. Marshall in town, Allison Pearson (Allison Janney), doesn't make a very good first impression with Mary. Mary's sister, Brandi (Nichole Hiltz), is struggling to have her tuition check refunded. And Peter even offers to help her financially, but Brandi has plans of her own.

An ATF agent is gunned down when an arms deal goes bad in El Paso, Texas. After awaking from a coma three years later, Wade Guthrie (Mike Doyle) is placed under WitSec protection as a star witness in a federal case. Wade's longing to be reunited with his past girlfriend leads WitSec into a dilemma as his partner and former girlfriend are, now, much more "acquainted." When Wade breaks free of US Marshal custody in order to see his former girlfriend, Mary and Marshall go into damage control mode and try to pacify an explosive situation. Also, Marshall relies on fate to link himself with a beautiful pharmaceutical saleswoman named Breck. Could this be love or just a flash in the pants? Meanwhile, Mary juggles the arrival of her fiancée's aunt, who is representing Raph's mother at the engagement party, and the new head at WitSec, Allison Person, who is making life difficult for Mary.

The FBI and WitSec are working together to protect a whistle-blower and it appears Mary has met her match. Mary and Special Agent Mike Faber (Steven Weber) butt heads when she arrives. She also finds out that Earl Donaldson (Geoffrey Blake) hasn't been told about all of his witness possibilities. When Donaldson finally chooses to enter witness protection, he jeopardizes his family's safety by telling them about his choice in a bugged house. The FBI's money laundering case then turns into a kidnapping investigation. WitSec and the FBI must race against time in the hopes of finding their witness who has actually taken matters into his own hands. Back in ABQ, Stan McQueen (Paul Ben-Victor) is helping an ex-boxer who is experiencing dementia. His fighting days long behind him, Ricky Dumont (Chad Coleman) believes he's still a heavyweight and that Stan is his old trainer. McQueen is burdened with the responsibility of breaking the bad news to Dumont.

Mary and Marshall travel to Providence, RI to observer their witness' trial when Dominic Difazzio (Louis Lombardi) becomes a star witness against his uncle and mob boss, Vincent Difazzio (Roy Costley). Given Difazzio's murderous track record with explosives, he's not one of Mary's favorites. After settling into his new life in New Mexico, Difazzio opens a local fish store and falls in love with a business neighbor named Naomi (Salli Saffioti). The only problem is that he doesn't know how to act around women, having never been on a date. He begs Mary for a little advice, and she grudgingly agrees. Mary says her goodbyes to Raphael (Cristián de la Fuente), only to run into him later when she accompanies Difazzio on his first date. Difazzio's date goes swimmingly and he tells Mary that he thinks he sees wedding bells in his future. But when Marshall and Mary go to Naomi's home to check on the lovebirds, they're surprised to find another man in the picture. Mary is handed the task of disclosing this information to Difazzio, hoping that his reaction isn't explosive.

Thirty-eight years ago, Frank Jergens' (Fred Ward) good side showed itself while he was robbing a bank. During the heist, he turned on the dirty cops who were helping him pull it off. After 38 years in the pen, Jergens is released to Mary and WitSec, but not before making friends and connections on the inside. Mary is very concerned about Frank's friend, Ray (Brian Goodman), who is indebted to two criminals who are eager to collect. And the more that Mary engages with Frank, the more she is reminded of her father. When Frank's friend Ray shows up at his door in dire straits, Frank must decide whether life on the outside is as good as it's cracked up to be. Meanwhile, Mary gets into trouble with WitSec administrators when they find out she's been abusing U.S. Marshal services to track down her sister. Marshall also has his hands full with an ex-writer who is falling apart with her life under protection. Once a promising writer, she now works in a copy store. Marshall goes the distance to get her a new job, but it might not be enough to calm her manic depression.

Marshall packs his bags for a witness escort out of state, leaving Mary behind in Albuquerque with FBI agent Mike Faber (Steven Weber), who's in town with a federal witness in need of protection from the Russian mob. Natalie Vickers (Alexandra Lydon) was once a part of a mortgage scam in Los Angeles, but now she's under Mary and Faber's watch. Seemingly too nonchalant about entering WitSec, Natalie has apparently attracted her accomplice and boyfriend to Albuquerque where they continue to operate real estate fraud and get caught in the process. The sexual tension between Mary and Faber rises, as the two of them work closely to stop Natalie from indulging in her criminal past. When Mary realizes that Natalie is doing it all for love, the boyfriend becomes a target of the FBI. Yet, Faber has another conflict to deal with, his superior officer in charge, Ike Dennis. After being promoted, Ike doesn't gel any more with the lower ranks. Mary softens to Faber as they mix business with pleasure. When Faber tells her how he really feels, Mary has to decide whether to open her heart to him.

Marshall's newest witness, Billy, was once a drug runner who witnessed a drug-deal gone deadly. Now taken under Marshall's wing, Billy is admitted into Albuquerque's best boarding school and urged to find friends who will accent his personality. If this wasn't enough of a load, Marshall's father, Seth shows up in Albuquerque on a sting operation called Operation Falcon that targets open warrants in the area. Marshall's straight-laced style contrasts with his father's maverick approach when he and Mary join Seth on his assignment. All hell breaks loose, when Marshall finds out Billy is involved in one of Seth's open-warrant apprehensions of a local criminal named Liam. Marshall seemingly works against his father in clearing Billy of any wrongdoing after finding out his witness' ulterior motive and relation to the crime.

Mia Cusato's (Laura San Giacomo) life turns upside down after she shoots and kills her own uncle (Michael Badalucco) who was attempting to "whack" her. But Cusato's future in WitSec is imperiled after she reveals a condition she's hidden from her family, a condition that may make her a questionable federal witness against her mob boss husband. Cusato and Mary hit it off when Mary discovers that their lives aren't so different from one another. Mary pulls all the stops to make sure that Cusato will remain protected while being interviewed by the attorney general in charge of the case. Marshall also has his hands full with a witness, Patrick (Travis Schuldt), who has been secretly been involved with his gay lover, Bob (Evan Gamble). Bob had relocated to Albuquerque after Patrick entered WitSec. Marshall must figure out a way to talk Bob into entering WitSec so that the two can live together. Jinx comes close to relapsing after Brandi reveals that she has a half-brother, fathered by Jinx's exhusband. But Mary helps her mother see what a great life she now leads in the hopes that Jinx will exorcise alcoholism's demons.

Judy Wagrowski (Karina Logue) is not your average accountant. She was once hired to keep track of the earnings in an illegal numbers racket in Pittsburgh, PA. After being raided by the feds, Wagrowski was turned over to WitSec and Mary becomes frustrated while trying to adjust to Judy's condition. Unable to lie and struggling with body language, Judy is given a job as an accountant's assistant while she waits to testify in the federal case. Mary walks a fine line as she instructs Judy to lie about her identity while in witness protection but to tell the truth in the courtroom once she's called upon to testify against her former boss, Larry Silvers (David Barrera). Marshall and Stan have their hands full as well when a current WitSec case becomes worried about her status as an immigrant in the States. Stan puts the pressure on the I.C.E. to renew her status, but he finds out that she may be involved in criminal activity.

WitSec receives a homeless but genius man who witnessed a homegrown bomb attempt and called the authorities, saving hundreds of innocent people. Walter Cooper (Esteban Powell) once lived off the streets of Seattle, Washington and consumed the many books at the library he lived in. But now, Mary and Marshall have to clean up this federal witness to that he's a credible witness for the U.S. Attorney's case. After cleaning up, Walter starts a job at a local Albuquerque bookstore where he's anything but receptive to the idea of an alarm clock, schedule, uniform and nametag. When WitSec uses a federal databank to track down a wanted local fugitive on the FBI's top ten list, Walter gets the idea to use the same method to find his birth mother. Adopted at a young age, Marshall and Mary find out where Walter's mother is located but must race against time to reconnect the two after they find out that she's in grave trouble. Scott seemingly falters with his addiction to gambling and Brandi lies to him about money she once promised him, now being tied up in a new business venture. Scott's presence in New Mexico may be coming to an end.

A teenager witnesses a bloody diamond store heist and she is immediately introduced into WitSec protection. However, her mother stays behind and lets her father accompany her. Mary takes the reigns and finds that Sabrina (Spencer Locke), the daughter, is torn between her separated parents who dislike one another. Her mother (Jessica Tuck) barely has any time to see her on planned visitations in Chicago, and her father (Christopher Cousins) thinks he can buy her love with cars and gifts. Mary is left to emotionally guide her as the "stepmother" and make sure that Sabrina makes her federal hearing to testify against the criminals. Meanwhile, Scott's gambling debts have caught up with him. He ends up in the hospital, badly beaten. He claims that he was mugged and robbed, but Brandi is beginning to think there are other factors at play.

Gabriel Adams (Joe Spano) witnesses a stripper's death when an argument between two sex traffickers spills out into the main club lounge. After his relocation to New Mexico, Gabe eagerly accepts his new role as a bartender. But the more that Mary checks on the priest leading up to his federal case, the more she finds out about his immoral past. Meanwhile, Stan must confront the daughter of a witness, Roy Keels, who has died under protection. She comes to find out more about her father, and after lying to her about his character, she finds out that he was nothing but a "petty" criminal. Mary's personal life is shaken up when Brandi and Peter announce that they're moving in together. And Marshall and Jinx pressure her to use her vacation days before they expire under new WitSec rules.

When a witness turns up dead, Mary ditches administrative leave to solve a 10 Little Indians story --- until her own brand of Post-Traumatic-Stress disorders her plan.

When a marijuana mastermind from the NorCal grasslands is unable to testify without the pot to neutralize his Severe Anxiety Disorder, Mary finds herself in the weeds.

When a hot mom-on-the-prowl witness is having problems with her brood, Mary soon finds herself playing babysitter to three difficult kids.

When a huge building collapse traps one of Mary's witnesses under the rubble and out in the open, his injury plays second to the threat of a professional assassin up on the roofs.

An Orthodox Jewish witness, whose estranged wife refuses to enter WITSEC, soon finds himself being followed by a mysterious man (guest star Richard Schiff) who answers to a higher authority.

When Stan's first witness turns up dead, with his badge amongst the bones, he becomes the chief suspect - forcing Mary & Marshall to dig deeper into a 20-year-old love triangle.

When a bridge collapses and plunges many to their death, the victims' families come after the engineer, who, once in WITSEC, attempts to clear his name no matter what the consequence.

When a series of murders seem to follow a new ex-con witness, including stabbing Mary's friend, she and Marshall have to figure out if he's the killer in time to save his next victim.

When Brandi is arrested by Agent O'Connor for drug trafficking and murder, Mary races to find a way out for Brandi and the sordid motive for his vendetta.

After briefing a new witness, whose testimony is contingent on visitation rights with his son, Mary learns the son is missing and sets out to find him.

The barely legal daughter of Mary's witness - a Miami racketeer - finds herself in a dangerous romance, just as Raph's mother visits to prepare an engagement feast for the surprised Mary.

Mary serves as Technical Advisor to The Marshals Service new training video, while Marshall arranges to get one of Witsec's charter members, a geriatric goodfella (Martin Landau) from the Philly Mob, to his son's funeral.

A world-class counterfeiter witness - whose girlfriend's infidelity moved her to confess - is suspect in the murder of another "friend", where even the clues seem forged, until Mary and Marshall realize they may be hiding a witness from the government itself.

When a Wall Street blue-blood rats on his boss' investment fraud and hides out in WITSEC, he volunteers to give up his great fortune, that is until a ransom from his past life comes calling.

The U.S. Government enlists WITSEC to hide one of Latin America's feistiest political activists and everything goes South when the budding Evita moves to the bad part of town.

U.S. Marshal Mary Shannon has a simple day ahead of her: pick up federally protected witness, Tasha Turischeva and get her situated - make sure her boyfriend picks up her ne'er do well sister, Brandi, from the train station - and survive the "surprise" party her mother has planned. However, when Mary learns that her witness, former hit-man Frankie Santoro's, a.k.a. Frankie "Nuts'" beloved son was murdered, all the above plans are relegated to the back burner. And for that she's almost grateful. Mary now finds herself faced with two problems: Was Jr. killed by someone from Frankie Sr.'s past? And how do you tell a mafia hit man that his only son was murdered? In order to keep Frankie from flying off in a blind vengeful rage, Mary has twenty-four hours to find and apprehend Jr.'s killer.

Leo Billups witnessed a drug-related murder that involved the Cruz family, one of the most powerful drug families in the country. It's a common enough story in WITSEC. What sets this one apart is that the person he saw murdered was his mother - and Leo is eight years old. When Leo enters WITSEC, no one's really sure what to do with him. His father can't go into the program, since he's one of the people Leo is testifying against. Mary asks another WITSEC family if they'll adopt him. When they agree, it seems like everything's going to work out perfectly. But Leo's father sues for custody, claiming the Federal government had no right to take Leo in the first place. Everyone knows he just wants Leo so he can keep him from testifying, but the law is on his side. Now, Mary must find a way to keep Leo away from his father and safe from the Cruz family.

Treena Morris is a con artist. Her particular skill: making men fall so crazy, head-over-heels in love with her that they'll do stupid things, like hand her a bag full of illegal conflict diamonds, and trust that she'll put them in the safe. Her current husband/mark is just that gullible. Unfortunately for her, before she can get away with half the diamonds, the Feds bust her. On the plus side, she's got more than enough dirt on her diamond-smuggler husband to make a deal. Mary is understandably disgusted when she's told she has to help this woman get a fresh start. She's even angrier when Treena gets engaged to one of the wealthiest men in Albuquerque, allows her engagement photo to be put in the paper, and refuses to call off the wedding despite Mary's warnings. Now Mary has to keep Treena safe from angry diamond smugglers and keep Treena's fiancé safe from her.

Horst Vanderhoff is an agent, which is difficult enough when you're dealing with actors, but Horst's job is a good deal more stressful. He's the go-between for a highly sought after hit woman known only as Lola. While trying to get her next job lined up, Horst is grabbed by the Feds and forced to give testimony against her in exchange for a chance at joining the Witness Protection Program. When Mary and Marshall show up at Perryville Prison to pick him up, he's justifiably nervous. He's also really irritating. Mary and Marshall set out toward the desert, convinced that the worst part of the job will be putting up with Horst for four hours. But when two killers ambush them, disable their car and wound Marshall, Mary has the much more difficult task of finding a way to keep all of them safe from Lola.

The Feds make a lot of promises in exchange for testimony, not all of them moral, and it's the Marshals who are stuck with the consequences. When art dealer Jay Arnstein goes into the witness protection program, he asks that his wife, Marci, be moved with him, as well as his mistress, Kay. The rules force Mary to keep this from Marci, a very sweet woman with whom she has an immediate bond. Things might have gone on like that forever, but when Jay is shot late one night at the gallery, everyone's secrets are forced out into the open.

Brothers Shane and Chris Worley have been Mary's witnesses for five years. They came into the program alone. Their parents are dead and Shane has been looking after Chris most of his life. Now it looks like all Shane's hard work is paying off. Chris is the star of his college basketball team and scouts are showing a lot of interest. The only problem is that Chris has started gambling and he's thousands of dollars in debt. Mary has to find a way to keep both brothers' lives from being destroyed by Chris' problem.

Iris McBride and her family are uprooted and forced to move to Albuquerque after she witnesses a gang shooting. Her father, a successful doctor, is livid when he learns that just because his daughter disobeyed him and went to the wrong part of town, he has to give up his practice. When Mary first meets the family she assures them that things will get better, but that's hard for them to believe when it'll take weeks for their furniture to get to them, they're living on a government stipend, and the only job the dad can get is at a free clinic. Mary has to keep Iris safe while she testifies, but the more difficult task is going to be keeping the family from falling apart.

Don and Ruth Ferguson are two of the nicest people you'll ever meet. They're devoted to the Catholic Church, they spend all of their time working at a homeless shelter and they've managed to recreate their lives after leaving everything behind to enter the Witness Protection Program. The only problem is: they can't stand each other, and they're too Catholic to divorce. Despite their misery, Ruth is devastated when she hears that Don has died in a traffic accident. As Mary investigates, she learns that the case is more complicated than it seems at first glance. The body in Don's car isn't Don, which begs the question: Could Don still be alive? If so, who has him?

Internal Affairs knows Rico Eps is one of the dirty cops on the Chicago force who isn't above planting evidence to make a case. But instead of taking him down, IA offers him a deal in exchange for investigating his fellow cops. Eps takes the deal, and when his partner finds out, he's almost killed. Staying on the police force obviously isn't an option. Eps has to leave everything behind and enter Witness Protection. When Mary and he meet there are immediate sparks, and even though she knows it's against the rules and an all-around terrible idea, she gives into temptation and sleeps with him. Afterwards, they part ways and have minimal contact over the next six months. Things might have continued like that, but when an Albuquerque cop with ties to Chicago is murdered, Eps quickly becomes the prime suspect. Mary has to both prove his innocence and keep their past from damaging her career.

Natasha, the mild mannered Ukrainian accountant, has gotten her new breasts, but her life becomes more complicated when she discovers that her new boyfriend is also in WITSEC.

After watching her mother audition for "Sweet Charity", Mary Shannon is kidnapped. Meanwhile in New Jersey, Brandi's boyfriend Chuck is also abducted and two FBI agents are killed. When the Feds working on Mary's kidnapping case suspect a connection between the two crimes, a nationwide manhunt goes underway for both Mary and Chuck.

Mary finally returns home after her harrowing kidnapping ordeal at the hands of the maniacal Spanky only to learn that Brandi's involvement with Chuck - who was shot dead right in front of Mary - not only Mary into that horrible mess in the first place, but could cost her her career. Mary tells Brandi that since she is under investigation for her part in the murder of two FBI Agents, she will have to report anything that Brandi tells her to the proper authorities. This is the one time that she cannot help her sister. Both Brandi and Jinx are stunned that Mary won't get involved but after what Mary has been through and what's at stake for her, it's clear that she is as serious as a heart attack.