A saber-toothed squirrel known as Scrat attempts to find a place to store his single acorn. Eventually, as he tries to stomp it into the ground, he causes a large crack in the ground that extends for miles before setting off a large avalanche which nearly crushes him. He barely escapes, but finds himself stepped on by a herd of prehistoric animals migrating south in order to avoid the ice age. Sid, a clumsy ground sloth is left behind by his family and decides to move on by himself but is attacked by two prehistoric rhinos whom he angered by ruining their meal. Sid is soon saved by Manny, an agitated woolly mammoth heading north, who fights the rhinos off and continues his path. Not wanting to be alone and unprotected, Sid follows Manny. Meanwhile, Soto, the leader of a pack of saber-toothed tigers, wants revenge on a group of humans for killing half of his pack, by eating the chief’s infant son, alive. Soto leads a raid on the human camp, during which the baby’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.
Later, Sid and Manny spot the baby and his mother near the lake, having survived her plunge. The mother only has enough strength to entrust her baby to Manny before she disappears into the water. After much persuasion by Sid, they decide to return the baby, 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 half-peak where his pack is waiting to ambush them.