Sid the Sloth is left behind by his friends and family when they migrate. He starts off on his own when he gets attacked by Rhinos. A mammoth called Manny saves him so Sid decides to follow him. Meanwhile a pack of sabre-toothed tigers attack a colony of humans. One tiger called Diego chases a mother and her baby to a waterfall. The mother jumps of the waterfall and dies, pushing her baby onto shore. Luckily, Manny and Sid take it to return it to the humans. Diego goes with them to lure them into a ambush. Diego gets saved by Manny and feels bad about leading them to their deaths so he tells them. They all concoct a plan and it works (sort of!) Diego gets injured and he tells them to leave him. Manny And Sid find the humans and return the baby safely. As they go to leave Diego appears good as new. They all migrate together.