Ice Age

A herd of prehistoric animals is migrating south to avoid a forthcoming ice age. Sid, a clumsy ground sloth, has been left behind. After being attacked by a pair of brontotheres, Sid is reluctantly rescued by Manny, a surly woolly mammoth who did not migrate with the others. Sid insists on following Manny, but the latter wishes to be alone and is annoyed by Sid's outgoing demeanor.
Some time before these events, a human tribe hunted and killed half of a Smilodon pack for their pelts. Soto, the leader of the surviving saber-toothed tigers, leads a raid on the human camp, intending to exact revenge by personally killing Roshan, the infant son of the chief. Soto has his lieutenant Diego bring the baby to him alive. Roshan's mother, however, flees with her son and leaps over a waterfall. Soto orders Diego to pursue Roshan and bring him back alive, threatening to kill him if he fails.
Manny and Sid encounter Roshan and his injured mother in the river at the bottom of the falls. The woman leaves the baby with them, but is washed away. Sid attempts to return the baby to the human settlement, but finds the camp deserted. Diego offers to take the baby, but a mistrustful Manny orders Diego to track the humans instead. Diego complies, but that night, he secretly sends word back to Soto that he is bringing both the baby and a mammoth for the pack to eat.
After several misadventures and bonding moments during their adventure, the trio travel through an ice cave and discovers human cave paintings. Though Manny experiences a flashback of his family being killed by humans, he has become deeply attached to Roshan. Diego, beginning to doubt his mission, changes sides after Manny saves him from a lava flow. He informs Manny and Sid about Soto's pack waiting to ambush them at Half Peak. Though the pair are skeptical, having learned they were set up, Diego convinces them their only chance is to trust him.
Manny, Sid, and Diego plot to trick the pack of cats and escape. This initially works, but Soto and two others manage to corner Manny at a cliff edge. Diego attacks Soto and saves Manny, but is severely injured in the process. Soto is thrown against an overhanging rock and is killed by falling icicles, which prompts the rest of his pack to flee.
As Manny and Sid grieve for Diego, he convinces them to leave him and return the baby to the humans before the mountain passes fill with snow. Manny and Sid catch up to the humans and Roshan reunites with his father, who gifts Manny the shell and bead necklace he had for his son. Diego, having survived his injuries, catches up just in time to say goodbye to Roshan, and the reunited trio, now best friends, head south towards warmer climes.