Plot
Sometime after the events of the first film, Manny, Sid, and Diego live in a valley that is surrounded by a high ice wall on all sides and is inhabited by many other mammals. The trio discovers that the ice wall is actually a dam that is barely holding a reservoir that could flood the valley if it fails. A vulture named The Lone Gunslinger tells them that there is a boat at the other end of the valley that may save them, but they must reach it in three days. A chunk of ice breaks off from the top of the dam, initiating their immediate evacuation. Manny is also struggling with the thought of possibly being the last mammoth alive.
Meanwhile, Cretaceous, a purple Metriorhynchus, and Maelstrom, a dark green Globidens—both frozen since the Mesozoic era escape due to melting ice and seek to eat all the mammals they can during the flood. On their journey to the boat, the trio meet Ellie, a mammoth who, due to being raised by a family of opossums, believes herself to be one. Sid invites Ellie alongside her adoptive brothers, Crash and Eddie, to join their journey to the boat, and they accept.
Following a risky encounter with Cretaceous and Maelstrom while crossing a pond, Ellie realizes she is a mammoth after remembering that as an orphaned calf, she was adopted by Crash and Eddie's mother. Manny begins to fall in love with her, but denies his feelings at first because he feels like he is replacing his deceased wife and child. Ellie also distances herself from Manny when he suggests "saving their species", taking it as an offense. They later reconcile.
The herd reaches the boat but it is behind a field of hot geysers. Manny, Sid, and Diego separate from Ellie, Crash, and Eddie when the two mammoths argue about which route is safest. Just as Manny, Sid, and Diego bypass the geysers, the ice dam fails, unleashing a flood upon the valley. Ellie, Crash, and Eddie, who took the safer yet longer way, are trapped inside a cave due to falling rocks. Crash and Eddie escape through a small hole and warn Manny, who rushes back to save Ellie. Cretaceous and Maelstorm ambush Manny underwater, but he tricks them into dislodging a boulder, thus freeing Ellie.
Manny and Ellie reunite with the others atop a boulder, but the water is still rising. Meanwhile, Scrat, after a series of misadventures to get back his acorn, climbs the adjacent glacial wall beside them and inadvertently creates a long crack when he punctures the ice. The crack widens into a gigantic fissure which splits open the wall and drains the floodwaters, saving everyone; in the process, Scrat falls within the fissure and is washed away.
A group of mammoths later appear from the fissure, proving to everyone that mammoths are not extinct. Manny initially lets Ellie go with the mammoth herd, but after some encouragement from Sid and Diego to move on from his past, he catches up to her, expressing his desire to stay with her. Manny, Sid, Diego, Ellie, Crash, and Eddie then venture out of the valley.
After falling into the fissure and nearly drowning, Scrat has a near death experience, entering a heaven full of acorns. Suddenly, he finds himself being "sucked back" just as he is about to reach a gigantic acorn. Scrat then discovers that he has been resuscitated by Sid. Scrat is enraged, believing that Sid stole his acorn, and proceeds to viciously attack him.
