Plot

When Sid's attempt to adopt three dinosaur eggs gets him abducted by their real mother to an underground lost world, his friends attempt to rescue him.

After the events of "Ice Age: The Meltdown", life changes for Manny and his friends: Scrat is still on the hunt to hold onto his beloved acorn, while finding a possible romance with female saber-toothed squirrel Scratte. Manny and Ellie have become an item and are expecting a baby, which makes Manny anxious to ensure that everything is perfect before their baby arrives. Diego is fed up with being treated like a house-cat and ponders the notion that he is becoming too laid-back. Sid starts to wish for a family of his own and steals some dinosaur eggs, which lands him in a strange underground world where his herd must rescue him while dodging dinosaurs, facing danger left and right, and meeting a one-eyed weasel known as Buck who hunts dinosaurs intently.—Theshapeshifter

With Ellie expecting their first baby, Manny is on his toes, struggling to make the snow-capped woods a safe place for his child after the events of Ice Age: The Meltdown (2006). In the meantime, once-ferocious saber-toothed tiger Diego experiences the midlife blues, while kind-hearted sloth Sid accidentally makes an extraordinary discovery: a triplet of shiny Tyrannosaurus Rex eggs. Determined to keep them, Sid finds himself stranded in an unknown, dangerous subterranean world, facing the wrath of mama T-Rex and a myriad of hostile creatures. Can the team save their reckless member? And is feral, one-eyed weasel Buck a friend or a foe?—Nick Riganas

When Sid takes three dinosaur eggs to the surface, he gets kidnapped by the mother dinosaur and gets taken to the underground dinosaur world. With the help of a friend, the group must do everything that they can to get Sid back while trying to survive.—RECB3

Ellie is pregnant, and Manny is determined to create a safe environment for her and their unborn child, haunted by the tragic loss of his previous family. Manny's over protectiveness, however, begins to push Diego away, as the saber-tooth tiger feels he's losing his edge and considers leaving the herd. Meanwhile, Sid, fearing abandonment, stumbles upon three eggs in an underground cavern. Ignoring Manny's warning to leave them, Sid adopts the eggs, which hatch into baby Tyrannosaurs the next morning. Elsewhere, Scrat becomes enamored with a flying saber-tooth squirrel named Scratte, though she is more interested in his prized acorn.

Despite Sid's efforts to raise the baby dinosaurs, their wild antics disrupt the herd and destroy the playground Manny built for his child. This angers Manny further. Matters escalate when the mother Tyrannosaurus arrives to reclaim her young. Sid refuses to return them, leading the mother to carry him and her babies underground. Manny, Ellie, Diego, and the possums give chase, discovering a massive subterranean world teeming with dinosaurs. They encounter a territorial dinosaur but are saved by Buck, a one-eyed, eccentric weasel who has been living in the jungle and hunting Rudy, a massive albino Baryonyx that cost him his eye. Buck agrees to guide them to Lava Falls, where the mother Tyrannosaurus is taking Sid and her young.

Meanwhile, the mother dinosaur gradually warms to Sid, though he later finds himself separated and pursued by Rudy. Sid narrowly escapes but ends up stranded on a rock drifting toward the perilous Lava Falls.

As the group closes in on Sid, Ellie goes into labor, and a pack of Guanlong dinosaurs triggers a rock slide that separates her from the others. Buck directs Manny and Diego to protect Ellie while he and the possums go after Sid. Diego rediscovers his courage while fending off the Guanlong and supports Ellie as she gives birth. Manny arrives just in time to meet their newborn daughter, whom they name Peaches. Buck, Crash, and Eddie rescue Sid in a daring effort, navigating dangerous flying reptiles and finally reaching Lava Falls just in time to save him. Sid, while overjoyed to reunite with the herd, is saddened that he couldn't properly bid farewell to the Tyrannosaurus family.

As the herd prepares to leave the dinosaur world, Rudy ambushes them. The mother Tyrannosaurus charges to their rescue, knocking Rudy off a cliff. With Rudy seemingly defeated, Sid says a heartfelt goodbye to the dinosaurs. Buck, initially planning to join the herd on the surface, changes his mind upon hearing Rudy's distant roar, indicating the predator's survival. Buck stays behind to continue his mission, sealing the passage to the lost world to protect his friends.

Back in their icy home, Manny and Ellie embrace their new life as parents to Peaches, while Diego decides to stay with the herd. Meanwhile, Scrat and Scratte briefly settle in the dinosaur world together, but Scratte's bossy nature drives Scrat to choose his acorn over her. Their ensuing tussle launches Scrat back to the surface, leaving Scratte and the acorn trapped in the dinosaur world.