Twenty thousand years ago, the Earth is overrun by freezing temperatures in an Ice Age that is sending all manner of critters scattering in the path of encroaching glaciers. When a lost human infant is discovered, an unlikely quartet of misfits forms to return it to its mother: a depressed woolly mammoth, a fast-talking sloth, an acorn-crazed squirrel and the devilish saber-toothed tiger. Before they can complete their mission, the reluctant compatriots will brave pits of boiling lava, dangerous caverns of ice, and even a traitorous plot within their midst.