Twenty thousand years before our era, when Scrat, a stubborn rodent, cracks the ice and triggers a new Ice Age, a vast herd of mammals gathers and begins migrating south. Manny, a solitary mammoth who always does things his own way, chooses to head north and is soon joined by Sid, a talkative little sloth looking for a protector.
Sid soon persuades Manny to take care of a human baby, Roshan, whom his mother entrusted to them before dying, and to return him to his father, a nomadic hunter who has headed north. Along the way, the two companions must protect Roshan from Diego, a saber-toothed tiger ordered by his leader, Soto, to capture the baby.