A saber-tooth squirrel named Scrat tries to store his acorn by stomping it into a patch of ice, but causes an avalanche in so doing, which he escapes, and leaves well enough alone until a mammoth foot stomps on him, along with herds of other animals, all of which are migrating south so as to escape the freezing winters brought on by the ice age. Among them are a surly mammoth named Manfred and a lazy ground sloth named Sid. Abandoned by his family, Sid attempts to migrate alone but runs afoul of two rhinos named Carl and Frank, who try to attack Sid but fail due to Manfred stepping in, only to leave Sid behind afterward. Sid stays with Manfred, referring to the mammoth as “Manny”, for protection, much to Manny’s dismay.
A tribe of humans camp near a waterfall, among them their leader, named Runar, with his wife Nadia and their infant son Roshan, before heading north to their settlement in the mountains. Soto, leader of a pack of saber-tooth cats, watches the humans with his trusted lieutenant, a cat named Diego. Soto, seeking revenge against the humans for wiping out half of his pack, intends to devour Runar’s son Roshan in vengeance and plans an attack for the next morning. The attack takes place, with Diego pursuing Nadia, but she escapes with Roshan, jumping over the waterfalls so that Diego returns to Soto with no baby; the pack agrees to meet at Half Peak, where Diego will produce the baby for Soto.