When a virus leaks from a top-secret facility, turning all resident researchers into ravenous zombies and their lab animals into mutated hounds from hell, the government sends in an elite military task force to contain the outbreak.

With the impending ice age almost upon them, a mismatched trio of prehistoric critters – Manny the woolly mammoth, Diego the saber-toothed tiger and Sid the giant sloth – find an orphaned infant and decide to return it to its human parents. Along the way, the unlikely allies become friends but, when enemies attack, their quest takes on far nobler aims.