20,000 years before, our planet is entering an ice age. All kinds of animals begin immigrating to the south, seeking more warm climates. Sid, a sloth who never stops talking is left behind sleeping while everyone else begins the journey to the south. Awaking, he meets Manny, a mammoth who travels to the north, and decides to follow him. When a humans camp is attacked by tigers, a woman takes her baby and jumps on a river. Before she drowns, the baby is rescued by Manny and Sid. The two animals decide to search for the father and return the baby to him.
