It is twenty-thousand years ago. The Earth is a wondrous, prehistoric world filled with danger, not the least of which is the beginning of the Ice Age. To avoid a really bad frostbite, the planet's majestic creatures - and a few small, slothful ones - begin migrating south. The exceptions are a woolly mammoth named Manfred, and a terminally lazy sloth named Sid (John Leguizamo). When Sid "adopts" Manfred (Ray Romano) as his protector, the mammoth tries everything he can to unload his newfound baggage. But that's only the beginning of his frustrations: Manny has been roped by Sid into helping reunite an abandoned human baby named Roshan (Goran Visnjic) with his family. They are joined by Diego (Denis Leary), a sinister saber-toothed tiger who befriends Sid and Manny, all the while seeing the infant as a tasty treat. As Sid, Manny and Diego make their way across the vast, ice-covered landscape, another creature, a prehistoric squirrel/rat known as Scrat, tries desperately to carry out his mission in life - to bury an acorn - which, alas, triggers calamitous events.