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(John Leguizamo), a sloth who never stops talking is left behind sleeping while everyone else begins the journey to the south. Awaking, he meets Manny(Ray Romano), 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. Diego(Denis Leary), one of the tigers that attacked the humans, comes also claiming the baby.