The film begins with a sabre-toothed squirrel (known as Scrat) who is trying to find somewhere to store his prized acorn. Eventually, as he tries to stomp it into the ground, he causes a large crack in the ground that extends for miles and miles and sets off a large avalanche. He barely escapes, but finds himself stepped on by a herd of prehistoric animals. The animals are trying to avoid the ice age by migrating south. Sid, a clumsy Megalonyx sloth left behind by his family, is attacked by two Brontops whom he angered. Sid is soon saved by Manfred ("Manny"), an agitated mammoth who fights them off. Not wanting to be alone and unprotected, Sid follows Manny. Meanwhile, Soto, the leader of a Smilodon pride wants revenge on a group of humans by eating the chief's baby son, Roshan, alive. Soto leads a raid on the human camp, during which Roshan's mother is separated from the rest and jumps down a waterfall when cornered by Soto's lieutenant, Diego. For his failure, Diego is sent to find and retrieve the baby. Sid and Manny spot Roshan and his mother near the lake, having survived her plunge. The mother only has enough strength to trust her baby to Manny before she disappears. After much persuasion by Sid, they decide to return Roshan (nicknamed "Pinky") but when they reach the human settlement, they find it deserted. They meet up with Diego, who convinces the pair to let him help by tracking the humans. The four travel on, with Diego secretly leading them to his pack for an ambush. While having small adventures on their way, they reach a cave where Sid and Diego learn about Manny's past and his previous interactions with the humans, in which his wife and son were killed, leaving Manny a cynical loner. At one time the group passes a flying saucer frozen in the ice, while Sid comes upon a display showing the evolution of sloths. At the end of the film, Diego, Manny and Sid battle Soto's pack and a short fight ensues. As Soto closes in for the kill on Manny, Diego leaps and stops Soto, who wounds Diego in the process. Manny, in vengeance, knocks Soto into a rock wall, causing several sharp icicles to fall on Soto, killing him. Manny and Sid manage to return the baby to his tribe, and Diego rejoins them, as the group begins to head off to warmer climates.
