The sub-zero heroes from the worldwide blockbusters Ice Age and Ice Age: The Meltdown are back, on an incredible adventure...for the ages. 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, Sid the sloth creates his own makeshift family by hijacking some dinosaur eggs; and Diego the saber-toothed tiger wonders if he's growing too soft hanging with his pals. 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 flora fauna run amuck--and meet a relentless, one-eyed, dino-hunting weasel named Buck.