In a melting world of ice and snow, Manny the woolly mammoth, Sid the sloth, and Diego the saber-toothed tiger discover their once icy paradise is on the brink of disaster. As the valley where they live begins to flood due to the rapidly melting ice, they find out from a vulture that there is a boat at the other end of the valley that can save them from a massive flood that threatens to drown all creatures in their habitat. Realizing the gravity of their situation, the trio decides to embark on a journey to find this boat, seeing it as their only chance for survival. Along the way, Manny wrestles with the fear that he might be the last of his kind, until he meets Ellie, a female mammoth who thinks she's a possum.

Throughout their journey, the group faces numerous challenges, including treacherous landscapes and menacing predators. They are also joined by Ellie's two possum "brothers," Crash and Eddie, who add their own mix of mischief and humor to the expedition. As they navigate through the dangerous terrain, the group learns to work together, discovering that family can come in the most unexpected forms. Their quest not only tests their strength and courage but also teaches them the value of friendship and teamwork in the face of adversity.