Plot

A saber-toothed squirrel (known as Scrat (Chris Wedge)) is trying to find somewhere to store his prized acorn. Eventually, as he tries to stomp it into the ice, he causes a large crack in the icy ground that extends for miles and miles and sets off a large avalanche. He barely escapes but finds himself stepped on by a herd of prehistoric animals.

The animals are trying to avoid the ice age by migrating south. Sid (John Leguizamo), a clumsy Megalonyx sloth left behind by his family, and chases the herd by himself. Sid is attacked by two Brontops (an extinct group of rhinoceros-like browsers related to horses) Frank (Stephen Root) and Carl (Cedric the Entertainer) whom he angered. Sid's feet were covered in feces, and he stepped on the Brontops food and made it inedible.

Sid runs away from the Brontops and into Manfred "Manny" (Ray Romano), an agitated mammoth who fights them off. Manny does not like animals who kill other animals for purposes other than eating them. The Brontops are vegetarians and had no intention of eating Sid and this is why Manny got angry and decided to fight the Brontops. Eventually, Manny picks the Brontops with his tusks and throws them off.

Manny is travelling in the opposite direction to the rest of the animal herds. Not wanting to be alone and unprotected (as the Brontops were still watching and waiting for the moment that Sid would be away from Manny), Sid follows Manny. Manny wishes to be alone and is constantly annoyed by Sid's outgoing demeanor.

For the night, Manny builds a shelter for warmth, but Sid claims to know how to make fire with a stick. That night a storm strikes, and Sid is not able to light a fire in the deluge. Sid takes Shelter with Manny and says that he was abandoned by his family when they migrated. Manny is a loner and avoids talking about his family. Scrat tries to secure his acorn in a tall tree but is stuck by lightning.

Meanwhile, Soto (Goran Visnjic), the leader of a Smilodon pride wants revenge on a group of humans by eating the chief's baby son, Roshan (Tara Strong), alive. Roshan is a toddler who has not yet started to walk. Soto says that the human chief is responsible for killing half of their pack and are wearing the skins of the massacred Smilodon to keep themselves warm. The other Smilodon include Diego, Zeke (Jack Black), and Oscar (Diedrich Bader). The humans have a small settlement in the forest and Soto wants to attack it.

Soto leads a raid on the human camp at dawn when all the humans were asleep. The humans are woken up by their pet dogs who sense the attack coming. During the chaos Diego enters the chief's tent and goes for Roshan. Roshan's mother picks him up from his crib and runs away from the tent, pursued by Diego. She separated from the tribe and jumps down a waterfall when cornered by Soto's lieutenant, Diego (Denis Leary). The humans eventually manage to fight off the Smilodon attack, and Soto calls his troops back as he sees Diego returning.

Sid and Manny spot Roshan and his mother near the lake, having survived her plunge. The mother only has enough strength to trust her baby to Manny before she disappears. Manny wants to walk away, but Sid says that they cannot leave the human baby alone. He observes smoke coming from the top of a hill and says that the human settlement would be there.

Soto is livid to learn that Diego lost Roshan over the falls. For his failure, Soto orders Diego to pursue Roshan and bring him back alive, threatening to kill him if he fails. Soto says that he will wait for Diego at Half Peak. Meanwhile, the humans pursue the Smilodon tribe.

Sid tries to climb up the waterfall to return Roshan, but he is physically incapable. Roshan slips and falls and is caught by Diego. Manny takes Roshan from Diego, who claims that he was returning Roshan to his settlement. Manny and Sid both don't trust Diego. After much persuasion by Sid, they decide to return Roshan (nicknamed "Pinky") but when they reach the human settlement, they find it deserted.

Diego convinces the pair to let him help by tracking the humans. He inspects the settlement and says that the humans went North less than 2 hours ago. Manny knows that Diego is the only hunter among them and has very good tracking skills. Manny knows that the humans have another settlement on the other side of Glacier pass and they need Diego to get them across the pass before the snow closes it down. Manny is suspicious of Diego and hence decides to not leave Pinky alone with Sid and Diego. The four travels on, with Diego secretly leading them to his pack for an ambush.

They have small adventures on their way. They fight a pack of dodos to get a melon for Pinky. Diego sends a message to Soto via Soto's spies that he is bringing a mammoth and to be prepared for an ambush at Half Peak. Manny starts bonding with Pinky and takes a liking to it. As they enter the mountains, Diego comes across the pack of humans passing the mountain to the other side. Diego purposefully changes direction and leads Manny and Sid through a different route to avoid the humans.

They reach a cave where Sid and Diego learn about Manny's past and his previous interactions with the humans, in which his wife and son were killed, leaving Manny a cynical loner. At one time the group passes a flying saucer frozen in the ice, while Sid comes upon a display showing the evolution of sloths.

The 4 get trapped in an under-ground volcano and Manny helps to save Diego's life by putting his own life at risk to rescue him. Diego begins to second-guess his mission and decides to change sides completely. Sid accidentally discovers fire when he draws cave art using a stone chalk. That night Pinky also takes its first steps and amazes everyone.

As they near the ambush at Half Peak, Diego reveals the plan to Manny. Diego says that he wants Pinky safe. Though the pair remain skeptical having learned they were set up, Diego successfully convinces them their only chance is to trust him.

Sid, Manny, and Diego concoct a scheme 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 jumps in to fight Soto and save Manny and is severely injured in the process. Soto is thrown against an overhanging rock, dislodging several large icicles, impaling and killing him; the rest of the pack flee.

As Sid and Manny 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 a shell and bead necklace. Diego, having survived his injuries, catches up just in time to say goodbye to Roshan, and the reunited trio, now best friends, head back south with the rest of the animals.