Ellie (Queen Latifah) and Manny (Ray Romano) are expecting their first child, and Manny is obsessed with making life perfect and safe for the family, since his first experiences as a husband and father went bad when his family was killed by hunters. At the same time, Diego (Denis Leary) finds himself unable to catch a cocky gazelle (Bill Hader) he has been stalking and decides to leave the herd, believing that he is losing his predatory nature as a tiger. Sid (John Leguizamo) grows jealous of Manny and Ellie and "adopts" three apparently abandoned eggs that he finds in an icy underground cavern and call them Eggbert, Shelly, and Yoko. Manny tells him to put them back (saying Sid would never make a good parent, saying, "First sign: Stealing someone else's eggs. Second sign: One of them almost became a omelette"), but Sid instead looks after the eggs, which hatch into baby Tyrannosaurus the next morning.
Although Sid tries his best to raise the three dinosaurs, their rambunctious behavior scares away all the other animals' young and ruins a playground Manny built for Ellie's baby. A female Tyrannosaurus, Momma, whose eggs Sid stole, soon returns and carries both Sid and her young underground, with Diego in pursuit. Manny, Ellie, Crash, and Eddie (Seann William Scott, Josh Peck) follow as well and discover that the icy cavern leads to a vast jungle populated by dinosaurs thought to be extinct. Here, an Ankylosaurus threatens the herd despite Diego's efforts to fend it off; they are saved from a further crowd of angry reptiles by an insane, one-eyed weasel named Buckminster, or Buck (Simon Pegg).
