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