As we rejoin the motley herd from the two previous Ice Age films, Manny the mammoth (Ray Romano) nervously pampers his expectant mate, Ellie (Queen Latifah), while his best friend, Diego the saber-toothed tiger (Denis Leary), bristles at the thought of Manny's domesticated existence. Meanwhile, Sid the sloth (John Leguizamo) continues to annoy everybody, and Scrat the rock rat is still trying to get that acorn. When Sid accidentally discovers an underground world populated by dinosaurs, he steals three dino eggs so that he, too, can become a "mommy." After the little ones hatch, their real mom comes to retrieve them, and snatches Sid as well, prompting the others to band together yet in order to rescue their friend.
by Perry Seibert synopsis