Manny the mammoth, along with his friends Sid the sloth and Diego the saber-toothed tiger, are living peacefully in their icy world. Manny is eagerly expecting the birth of his first child with Ellie. Sid, feeling left out, discovers three abandoned eggs in an underground cavern and decides to raise them as his own. The eggs hatch into baby dinosaurs, and their mother, a massive Tyrannosaurus rex, soon arrives to reclaim them, taking Sid and her babies to a mysterious underground world where dinosaurs still roam.

Determined to rescue Sid, Manny, Ellie, Diego, and Ellie’s opossum brothers Crash and Eddie venture into the lush and perilous underground world. Along the way, they encounter Buck, a one-eyed, crazed weasel who becomes their guide through the dangerous environment. As they journey deeper, they face various threats, including carnivorous plants, roaring predators, and treacherous terrain, putting their bravery and friendship to the ultimate test.