Twenty-thousand years ago the Earth was a wondrous, prehistoric world filled with danger, not the least of which comes at the beginning of the Ice Age. To avoid really bad frostbite, along with the possibility of extinction, the planet's majestic creatures — and a few small slothful ones — begin migrating south. The exceptions are a woolly mammoth named Manfred, a high plains drifter who does things his own way, and a terminally lazy sloth named Sid, who enjoys doing nothing his own way.

When Sid "adopts" Manny as his protector, the mammoth tries everything he can to unload his newfound baggage. But that is only the beginning of his frustrations: Manny has been roped by Sid into helping reunite an abandoned human baby named Roshan with his family. This unlikely crew is joined by Diego, a sinister saber-toothed tiger who befriends Sid and Manny, all while seeing the infant as a tasty treat.

During their journey a prehistoric squirrel/rat called Scrat tenaciously chases his beloved acorn through their path. This all unfolds while our three heroes, on their quest to return the human child, narrowly escape avalanches, battle for food, careen through a roller coaster of ice funnels and traverse ice bridges over lakes of volcanic lava. Throughout their mission they become the weirdest herd of this, or any age.