Handlung

Scrat (Chris Wedge), the saber-toothed squirrel climbs a glacier with great difficulty and effort. At the top he finds his beloved acorn embedded in the ice and proceeds to extract it forcefully. He accidentally opens a hole in the ice, causing water to spurt out.

The world of ice is slowly melting. The creatures of the Ice Age are all shown enjoying themselves on slides and pools made by the melting ice, even though some mammals are worried that this is now a global warming phenomenon. Among them are Manny (Ray Romano) the woolly mammoth, Sid (John Leguizamo) the ground sloth, and Diego (Denis Leary) the Smilodon.

Sid opens a small day camp, where none of the younger creatures take him seriously, nor do Manny and Diego, which leaves Sid seeking a daring deed. Sid believes that he is an equal member of the herd and deserves respect. Manny narrates a story to the mammal kids but is only reminded that he does not have a family of his own.

Fast Tony (Jay Leno), a giant armadillo and a local con artist is claiming that the earth will flood and that the bark and reeds which he sells are needed to stay alive. Manny intervenes and tries to calm the mammals down. He says that the ice is not about to go anywhere. But the mammals are skeptical as mammoths are themselves near extinction and as such their survival instincts cannot be trusted.

Manny dismisses the idea of a global ice meltdown but is distracted when he sees that Sid is trying to high dive from a giant waterfall coming from a tall glacier. Manny goes with Diego to the top of the glacier to save Sid from his act of daredevilry, they see that the pleasant weather has caused the ice shelves to melt. The water has now accumulated into a giant lake, which is being kept from destroying the valley only by the glacier walls, which have formed a dam. Manny, Diego and Sid come down from the glacier and warn the mammals that Tony is right, and that the valley is going to flood soon.

A Lone Gunslinger Vulture (Will Arnett) warns the animals that a giant tree can act as a boat and save them if they make it to the end of the valley within three days' time, and all soon set out to find it. A chunk of ice breaks off from the top of the glacier dam, initiating their immediate evacuation. Manny is also struggling with the thought of possibly being the last mammoth alive. Manny knows that in a short time the valley will be a mile underwater and so the only hope is at the end of the valley.

As the animals begin their journey, Sid sings three songs to tease Manny about Mammoths being "extinct". During the evacuation, a glacier which contains two sea reptiles from the Mesozoic era, Cretaceous, an Ichthyosaur and Maelstrom, a Pliosaur, breaks off. Cretaceous and Maelstrom seek to eat all the mammals they can during the flood.

Manny believes that he heard a fellow mammoth and chases the sound, only to find that it was a different mammal bellowing into a tree trunk, which made it sound like a mammoth. Manny is depressed and wants to be alone for a little while. When Manny briefly is separated from them, Diego and Sid encounter two mischievous possums named Crash (Seann William Scott) and Eddie (Josh Peck) who drive them nuts by playing whack-a-mole with them. Manny is despondent about being the last mammoth alive, but is surprised & ecstatic when he encounters Ellie (Queen Latifah), a female mammoth who believes she is an possum and Crash and Eddie's sister.

Sid invites her to tag along with the group to escape the flood, and she brings her brothers. Ellie argues that they will never make it in time if they only travel at night and the trio can protect them from predators during the day. Diego is very hostile to Crash and Eddie.

Meanwhile, Scrat is still at the glacier wall trying to retrieve his acorn that eludes him.

Manny tries to convince Ellie that she is a mammoth, but she is adamant that she is a possum. Manny says that they have similar footprints and even their shadows match. Ellie says that Manny must be part possum. After a perilous ordeal with Cretaceous and Maelstrom while crossing a pond, Sid finds out that Diego is afraid of water, prompting him to encourage Diego to admit and face his fears. Diego insists that "fear is for prey", so Sid points out that Diego is behaving as if he is the water's prey. Manny begins to fall in love with Ellie but denies his feelings at first because he feels like he is replacing his deceased wife and child.

They discover an area which Ellie recalls as the place where she was adopted. She finally realizes she is a mammoth but distances herself from Manny when he suggests "saving their species".

Ellie and Manny ultimately make up when they must co-operate to save the group when the ground cracks under their feet. Sid is kidnapped by a tribe of mini sloths who believe Sid to be a God. Sid lights a fire for them, and believes that he has finally found respect, but they plan to sacrifice him by tossing him into a volcano. Sid narrowly escapes. The herd takes a break for the night.

The next morning, Sid tells the others about his experience, but none are convinced. The group finds the boat behind a field of hot geysers. Manny, Sid, and Diego separate from Ellie, Crash, and Eddie when the two mammoths argue about which route is safest.

Just as Manny, Sid, and Diego bypass the geysers, the ice dam fails, unleashing a flood upon the valley. Ellie, Crash, and Eddie, who took the safer yet longer way, are trapped inside a cave due to falling rocks. Crash and Eddie escape through a small hole and warn Manny, who rushes back to save Ellie. Manny saves Ellie from drowning as she is caught in a cave (due to falling rocks), while Diego overcomes his fear of water to save Sid. Cretaceous and Maelstrom arrive, but due to Manny's quick thinking, they are finished off by a rock which falls on them, killing them both. The other animals are at the mercy of the water currents. Manny and Ellie reunite with the others atop a boulder, but the water is still rising.

Meanwhile, Scrat climbs up the adjacent glacier wall and at the top sticks the acorn he has into the ice. He inadvertently creates a long crack when he punctures the ice. The crack widens into a gigantic fissure which splits open the wall and drains the floodwaters, saving everyone; in the process, Scrat falls within the fissure and is washed away. A group of mammoths later appear from the fissure, proving to everyone that mammoths are not really extinct. Manny initially lets Ellie go with the mammoth herd, but after some encouragement from Sid and Diego to move on from his past, he catches up to her, expressing his desire to stay with her. Manny, Sid, Diego, Ellie, Crash, and Eddie then venture out of the valley.

Sid encounters the mini-sloths again - they believe Sid stopped the flood and invite him to be their leader.

Diego, surprised to see the mini-sloths are real, convinces Sid to stay with the others, reluctantly admitting that Sid is a vital part of their 'herd'. Scrat has a near death experience after falling into the fissure. He enters a heaven full of acorns. Suddenly, he finds himself being "sucked back" just as he is about to reach a gigantic acorn. Scrat then discovers that he has been resuscitated by Sid. Scrat is enraged, believing that Sid stole his acorn, and proceeds to viciously attack him.