Back when the Earth was being overrun by glaciers, and animals were scurrying to save themselves from the upcoming Ice Age, a stupid sloth named Sid, a woolly mammoth named Manny, a saber-toothed tiger named Diego, and an acorn-loving saber-toothed squirrel named Scrat are forced to become unlikely heroes. The four reluctantly come together when they have to return a human child to its father while braving the deadly elements of the impending Ice Age.

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. Diego, one of the tigers that attacked the humans, comes also claiming the baby.

While Skrat the Saber Toothed Squirrel is still ineffectually trying to regain his precious acorn, the misfit trio of Manny the Mammoth, Sid the Sloth and Diego the Sabretooth Tiger have settled down in an isolated valley with numerous other animals. However, the trio discover that the ice wall surrounding the valley is barely holding back a massive body of water behind it and it's melting state threatens to break and flood the valley. With their one chance of survival being a boat at the other end of the valley, the trio follow the desperate exodus there. Along the way, they meet Ellie, a female mammoth who is convinced she's a opossum like her brothers. While the strange group continues the trek, they must learn to get along even as Manny struggles to find some connection to this strange female who may be the only other one of his kind.