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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.