The movie begins with Scrat once again in pursuit of the elusive acorn. It ends up landing on a small cliff and then disappearing near a tree. As he rounds the tree, he sees Scratte, and their theme song by Al Green, “You’ll Never Find,” starts playing as they both dance around who gets the acorn. Eventually, both fall off a cliff, and Scratte grabs the acorn and opens her arms like a parachute (flying squirrel-style) and leaves Scrat falling to the floor.
Manny (Ray Romano,) the two opossums, and Sid (John Leguizamo) are all running in Ellie’s (Queen Latifa’s) direction with a large leaf of hot water since she is pregnant, and Manny thinks she’s in labor. He slips courtesy of Sid and the water is lost. Ellie then tells him it was just a kick and to stop overreacting. Manny then shows her a play yard he made for the baby-to-be along with an ice mobile featuring both of them and the baby. Sid is somewhat upset that he is not on the mobile, and the opossums offer to add him to their smelly one. They all are wondering where Diego is.
Diego (Denis Leary) is stalking an antelope in the grass seemingly back to his predator roots. He chases the antelope until his sight blurs and he’s out of breath. The antelope laughs at this lack of energy, and Diego feels rather embarrassed. When he runs into Ellie and Manny, Ellie urges Manny to talk to him, and Diego announces that he will leave the “herd” since he and Sid will be out of the new “herd” once the baby arrives. He also tells Sid that Sid is basically being kicked out as well and to not go with Diego either. Sid is rather saddened by this and resolves to make his own herd.
As Sid ventures out, he falls into the ice and runs into three large eggs which he takes with him (Eggbert, Shelly, and Yoko.) He nearly loses all three, and Ellie saves one from falling off a cliff. Manny warns him to put the eggs back, but Sid insists that they were abandoned, and he must now take care of them. He guards them through the rain and sees shadows of small animals in the eggs when the sun comes out.
The next morning, Sid wakes up, oblivious to the fact that all the eggs hatched, and three baby T-Rexs are now attached to him. Without Manny’s permission, he takes them to the play yard Manny made and unwillingly allows tons of other kids in there. Chaos ensues, and the playground and mobile are destroyed. Manny yells at Sid and tells him that his “kids” are dangerous and need to leave. This is followed by the T-Rex mother showing up and taking all three of her children and an unwilling Sid back down under the ice into the world of dinosaurs.
When Ellie and Diego see where Sid has gone, they decide to go rescue him. Manny is against this, but follows reluctantly. Once they are under the ice, several dinosaurs attack them, and they are saved by a delusional recluse weasel named Buck (Simon Pegg.) Buck, who lost his eye to a colossal dinosaur named “Rudy” and took one of Rudy’s teeth as a knife souvenir, offers to help the group find Sid but points out that it’s very dangerous without his help. He demonstrates his skills by saving them all from several dinosaurs and a large predatory plant.
In the meantime, Sid is arguing with the mother dinosaur about how to raise the babies and tries to make them vegetarians, which doesn’t work. Although he fails at being the “natural” mother, the mother T-Rex allows Sid to tag along, and even hides him from “Rudy” who prowls at night looking for dinosaurs and other animals to eat.
Next, Sid’s rescue party encounters a cave of nitrous oxide that they must navigate through without laughing themselves to death. During Diego and Manny’s laughing experience, they admit to feeling badly about the herd possibly breaking up. Ellie hears all of this and realizes what they have been discussing beforehand. They find evidence that Sid is still alive, and Diego and Buck continue to track him into a large cavern.
As they all try to navigate the cavern, Ellie goes into labor (using the secret word, “peaches,” to alert Manny,) and she is stuck on a ledge with several small predatory dinosaurs headed her way. Buck splits up the group, Manny and Diego will help Ellie, and Buck and the opossums will save Sid. Diego goes back to his roots and successfully maims most of the dinosaurs who approach Ellie while Manny seals an entrance preventing any more to come near her. At the end of the battle, Ellie has given birth to a female baby that they name Peaches.
Buck and the opossums save Sid by flying a Pterodactyl above a lava river where Sid has become trapped by Rudy. They fly him to safety near the rest of the herd, and all are about to leave until Rudy emerges. Diego, Manny, Buck, and Sid trap Rudy and nearly escape, but Rudy breaks free. Just as he is about to eat Sid, the mother T-Rex shoves Rudy off a cliff for nearly killing the protector of her eggs. Sid says good-bye to her and the babies.
As the entire herd plus the new baby head back up to the ice, Buck nearly joins them until he hears Rudy roaring from below, and he tells Diego to protect the herd and then destroys the path for any dinosaurs or himself to follow the herd. The movie ends with the herd staying together and a new mobile is made with all the herd’s images on it.
Side story- Scrat and Scratte end up pursuing the acorn throughout the film, and they literally tango into love. At the end of the film, we see her ordering him around, and he looks out their love nest to see the acorn. He leaves her, and their fight for ownership of the acorn begins again.
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