Scrat is still trying to nab the ever-elusive nut and in the process may find his 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 very close encounters with dinosaurs, battle flora and fauna, and where they meet a relentless, one-eyed, dino-hunting weasel named Buck.