Scrat's epic pursuit of the elusive acorn catapults him into the universe where he accidentally sets off a series of cosmic events that transform and threaten the Ice Age World. To save themselves, Sid, Manny, Diego, and the rest of the herd must leave their home and embark on a quest full of comedy and adventure, travelling to exotic new lands and encountering a host of colorful new characters.
Set 20,000 years back, our planet Earth is entering an ice age. All animals are immigrating to the south in search of warmer climates. The film starts off with Scrat trying to find a secure location to hide his prized acorn. While trying to do so, he results in causing an avalanche. Though he escapes, he ends up landing on a herd of prehistoric animals. These animals are escaping to the south to avoid the ice age. The clumsy Sid is left behind as he had been sleeping. He is attacked by two Brontops but is saved by the agitated Manfred (Manny). Hence, Sid follows Manny as he is alone and unprotected.
On the other hand, a group of tigers headed by Soto attack a human camp as part of revenge. The chiefs son, Roshan is separated from his mother and the rest of the humans when his mother tries to jump over a waterfall. Roshan is rescued by Sid and Manny and decides to return the baby to the camp. However, the humans are gone when they reach the camp. Diego, Sotos lieutenant is sent to find the baby and bring back. He meets Sid and Manny and convinces them to help find the humans. Hence, the four start traveling where Diego leads them to a secret trap.
They reach cave where Sid and Diego come to know about Mannys past and his experience with humans. The humans had killed his wife and son leaving Manny a cynical loner. Towards the end of the movie, Diego, Sid and Manny fight with Sotos pack. Surprisingly, when Soto nears Manny, Diego jumps in to stop Soto thereby wounding Diego in the process. To take revenge, Manny knocks down Soto who is killed by sharp icicles falling on him. Somehow, Manny and Sid succeed in returning the baby to the tribe. Diego joins both of them and the three head towards warmer climates.