The sub-zero heroes from Ice Age are back on an a new incredible adventure. Scrat is still trying to nab the ever-elusive nut (while, maybe, finding true love); Manny and Ellie await the birth of their mini-mammoth; Diego wonders if he is going too soft hanging with his pals and Sid gets into trouble when he creates his own makeshift family by hijacking some dinosaur eggs. On a mission to rescue the hapless Sid, the gang ventures into a mysterious underground world, where they have some close encounters with dinosaurs, battle floura and fauna, run amuck and meet a relentless, one-eyed, dino-hunting weasel named Buck.