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 the saber-toothed tiger wonders if he's growing too "soft" hanging with his pals and Sid the sloth 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 flora and fauna, run amuck - and meet a relentless, one-eyed, dino-hunting weasel named Buck.