In the opening scene, Scrat, the saber-toothed squirrel climbs a glacier but accidentally opens a hole in it. 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; among them the three protagonists of the first film: Manfred, Sid, and Diego. Sid opens a day camp, where none of the younger creatures take him seriously, nor do Manfred and Diego, which leaves Sid seeking a daring deed. Fast Tony , 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. Manfred dismisses the idea, but is distracted when he sees that Sid will try to high dive from a giant waterfall; as Manfred goes with Diego to the top of the waterfall to save Sid from his act of daredevilry, they see that the pleasant weather caused the ice of the valley to melt, and it is kept from destroying the valley only by the glaciers, which have formed a dam.
The next morning Sid tells them his experience but none are convinced. They find a field of hot geysers, which separate Manfred, Sid, and Diego from Ellie and her brothers.When the flood comes, Manfred saves Ellie from drowning as she is caught in some rocks, while Diego overcomes his fear of water to save Sid. Cretaceous and Maelstrom arrive, but due to Manfred's quick thinking, they are finished off by a rock which falls on them, killing them both.