Scrat is still trying to nab the ever-elusive nut and maybe finding true love; Manny and Ellie await the birth of their mini-mammoth; Diego the saber-toothed tiger wonder if he's growing too 'soft' handing 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 very close encounters with dinosaurs, battle flora and fauna, run amuck, and meet a relentless, one-eyed, dino-hunting weasel named Buck.