In the opening scene, Scrat, the saber-toothed squirrel climbs a glacier to bury his acorn, but accidentally opens a hole in it, 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; among them the three protagonists of the first film: Manny, Sid, and Diego. 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. 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. Manny dismisses the idea, but is distracted when he sees that Sid will try to high dive from a giant waterfall; as Manny goes with Diego to the top of the waterfall to save Sid from his act of daredevilry, suddenly the ice under Diego’s feet begins breaking, causing him to bolt in fear, this also made Sid figure out that Diego is afraid of the water. The herd figure that the pleasant weather has caused the ice shelves to melt, and it is kept from destroying the valley only by the glaciers, which have formed a dam.
A lone vulture 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. 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 and Maelstrom, breaks off.