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After the events of Ice Age: The Meltdown, life begins to change for Manny (Ray Romano) and his friends: Scrat is still on the hunt to hold onto his beloved acorn, while finding a possible romance in a female sabre-toothed squirrel named Scratte. Manny and Ellie (Queen Latifah), having since become an item, are expecting a baby, which leaves Manny anxious to ensure that everything is perfect for when his baby arrives. Diego (Denis Leary) is fed up with being treated like a house-cat and ponders the notion that he is becoming too laid-back. Sid (John Leguizamo) begins to wish for a family of his own, and so steals some dinosaur eggs which leads to Sid ending up in a strange underground world where his herd must rescue him, while dodging dinosaurs and facing danger left and right, and meeting up with a one-eyed weasel known as Buck (Simon Pegg) who hunts feathered dinosaurs intently.