The film begins with a squirrel known as Scrat, who is trying to find a location to store his prized acorn. Eventually, as he tries to hide it, he causes an avalanche.

In the beginning, most animals are trying to avoid the ice age by migrating south. Sid, a clumsy ground 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 off the two Brontops. Not wanting to be alone and unprotected, Sid follows Manny.

Meanwhile, Soto, the leader of the Smilodon pride wants revenge on a group of humans by eating the chief's son, Roshan, because the humans had wiped out half of his pack. During the attack by the Smilodon pack on the humans, the baby's mother evades the cats who are pursuing her by jumping into a waterfall. With that, Soto orders Diego to find the baby and bring it to him alive.

Sid and Manny spot Nadia near the lake having survived her fall, having only enough strength to trust her baby to Manny before she disappears. After much persuasion by the sloth, they decide to return the baby (nicknamed "Pinky") but when they get to the human camp, the humans are gone. Diego convinces the pair to let him help by tracking the humans. The four travel on, with Diego secretly leading them to an ambush. Soon they reach a cave where Sid and Diego learn about Manny's past and his previous interactions with the humans.

At the end of the film, Diego, Manny and Sid battle Soto's pack and a short fight ensues. Diego fights against Soto, and Soto knocks Diego out. 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 wall of rocks, where icicles fall and kill Soto. Manny and Sid manage to return the baby to his tribe, and Diego rejoins them, as the group begin heading off to warmer climates