Having survived the previous meltdown of their ice age world that's now returned to its snowy status, Manny the mammoth (voice of RAY ROMANO) and his mate Ellie (voice of QUEEN LATIFAH) are expecting their first child, all of which has left Diego the saber-toothed tiger (voice of DENIS LEARY) and Sid the sloth (voice of JOHN LEGUIZAMO) feeling like third and fourth wheels.
Having lost his edge and stamina in hunting down prey, Diego thinks it's time for him to move on, while Sid feels he's losing his unorthodox family and longs to have one of his own. When he ends up falling through a layer of ice down into a hidden cave, he "adopts" three eggs, unaware that they're about to hatch into baby T-Rexes.
Yet, rather than eat him, the three baby dinos (that are bigger than him) view Sid as their mother, a development that doesn't sit well with their real and significantly larger mom who carries them and Sid back into her underground tropical world where all manners of other dinosaurs exist.
Despite Ellie being near labor at any moment, she, Manny and Sid, along with possums Crash (voice of SEANN WILLIAM SCOTT) and Eddie (voice of JOSH PECK), set out on a rescue mission. Their unexpected guide into this prehistoric world is Buck (voice of SIMON PEGG), a one-eyed swashbuckling weasel who wants to get his revenge on an enormous T-Rex he's nicknamed Rudy.
While saber-toothed squirrel Scrat continues his obsession with getting his paws on the ever-elusive acorn and must contend with competition from female saber-toothed squirrel Scratte, the unlikely group of mammals set out to find and rescue Sid, all while trying to avoid the monstrous carnivore that would like to make lunch of all of them.