Synopsis: Manny the mammoth starts to worry when his daughter Peaches announces her engagement. Still unlucky in love, Sid the sloth volunteers to plan the couple’s wedding. To Manny’s dismay, nothing can stop the upcoming nuptials, except maybe the end of the world. When Scrat accidentally launches himself into outer space, he sets off a chain reaction that sends an asteroid hurtling toward Earth. Now, the entire herd must leave home to explore new lands and save itself from Scrat’s cosmic blunder.
During the Herd’s journey to the crash site, Manny and Ellie try to manipulate Peaches to stay with them forever by making her think its her idea, but Peaches finds out and scolds them, while Buck discovers that the asteroids have electro-magnetic properties, theorizing that if a huge quantity of smaller asteroids should be gathered and launched into orbit, they could attract the main asteroid away from Earth, stopping it. After facing several obstacles and the interference of the dromaeosaurs, The Herd arrives at “Geotopia”, a community of immortal animals formed inside one of the asteroids that have fallen long ago, where Sid meets Brooke, a female ground sloth who instantly falls in love with him.
However, Shangri Llama, the leader of Geotopia, refuses to cooperate with Buck’s plan to use the city’s crystals in order to stop the asteroid, as they are the key to the residents’ longevity. After seeing how much sad Peaches is, thinking that their last chance of survival is gone, Manny and Ellie regret their actions. Sid inadvertently destroys the entire city when he attempts to remove one of the crystals to present Brooke with, immediately aging them to their real ages and revealing their true crone-like appearances, much to Shangri Llama’s anger.