In a bid to perfect one of their adventurous stunts, Crash and Eddie inadvertently trigger an avalanche that destroys the gang's summer dwelling. This reckless act angers Manny, Sid, Diego, and Ellie, who express skepticism about Crash and Eddie's ability to survive independently. Eager to prove them wrong, the duo decides to sneak away from their temporary camp while the others sleep.
As morning breaks and the gang realizes the possums are missing, Ellie insists that they embark on a search. Meanwhile, Crash and Eddie find a way into the Lost World, a realm filled with dinosaurs, where they reunite with their old ally Buck. Buck heroically saves them from a pack of raptors and informs them of the escape of Orson, a Protoceratops who was once bullied due to his impressive intellect. Orson now seeks to take control of the Lost World.
Buck tries to guide Crash and Eddie back home, but they discover that the entrance has been blocked by a massive boulder placed by Orson. Seeking refuge, Buck takes the possums to his shelter and recounts his past as part of a team that created a watering hole, a safe place for animals. Buck reveals that Orson rejected the opportunity to join them, believing instead in a survival-of-the-fittest mentality, which ultimately led to his banishment to an island. During his exile, Orson learned to control two raptors with fire.
When Buck's hideout is discovered by the raptors, they are saved by Zee, a zorilla and former member of Buck's team, who employs a gas to incapacitate the predators. However, Orson and his raptor army soon retaliate, attacking the watering hole while Buck and Zee urge the other animals to evacuate. Despite their past issues, Buck and Zee put aside their differences and, with the help of Crash and Eddie, seek assistance from their old friend Momma, a fearsome Tyrannosaurus.
The confrontation escalates as Orson's forces attack again. Buck willingly sacrifices himself by getting captured, enabling the others to escape and regroup. As they strategize to rescue Buck, Ellie, Manny, Sid, and Diego discover the entrance to the Lost World and team up with Momma to reach the watering hole where Buck is being held captive. Orson plans to use Buck as a warning to discourage dissent.
Faced with a monumental choice, Buck attempts to reason with Orson, advocating for peaceful coexistence. Yet, Orson stubbornly dismisses these pleas. Realizing that fire is the key to controlling the raptors, Crash and Eddie ingeniously create their own flames, redirecting the raptors to chase Orson instead, ultimately saving the Lost World from chaos.
After the battle, Ellie, Manny, Sid, and Diego express their regret for underestimating Crash and Eddie and invite them to return home. However, the possums declare their desire to remain in the Lost World alongside Buck and Zee. Though they are sad about the separation, Ellie beams with pride at the newfound maturity of Crash and Eddie and bids them farewell, knowing they will visit often.
