A herd of prehistoric animals are migrating south to avoid a forthcoming ice age. Sid, a clumsy ground sloth, is left behind by his family and chases the herd by himself, but is attacked by an angry pair of brontotheres after he interrupts their meal. Sid is reluctantly rescued by Manny, a surly woolly mammoth who did not migrate with the others; Sid insists upon following Manny, but the latter wishes to be alone and is constantly annoyed by Sid's outgoing demeanor.