Therefore we will not fear, though the earth should change,
though the mountains shake in the heart of the sea.
Scrat, still trying to catch that elusive acorn, chases it to the core of the Earth, where he sets into motion the seismic cataclysm known as the continental drift. When the rift separates Manny the mammoth, Sid the sabre- tooth tiger and some of their friends from Ellie and the rest of the herd, their patch of territory drifts out to sea where they encounter a band of pirates and other adventures while trying to re-unite with the herd.
Meanwhile Ellie and her daughter move toward a land bridge which they hope will enable them to be all together again. Silly but fun, and with great family values and theme of coping with change. The 3-D is especially used to good effect—some might say too much—though I still believe that you will not miss much, and save a lot, by going to the flat version.