It is a spin-off of the Ice Age series
Concerned about the two possums Crash and Eddie, Ellie, their wooly mammoth sister, calls for them, but they refuse to listen, calling themselves independent possums. They accidentally cause an avalanche and destroy the gang's summer camp. Manny, Ellie's husband, scolds Crash and Eddie for their recklessness and irresponsiblity. Feeling guilty, Crash and Eddie sneak out and leave the new camp. Angry with Manny, Ellie insists that they leave to go find them. Crash and Eddie stumble upon the entrance to the Lost World, a land where dinosaurs survive, and quickly get in trouble with two raptors trying to eat them. Breaking away, they run into Buck Wild, who helps them escape and tells them that a dwarf Triceratops named Orson, who has a big brain, has escaped from exile and has come to conquer over the Lost World. Buck tries to get Crash and Eddie back to their home, but figures out a boulder is covering the entrance to the Lost World. Orson appears and says that he sealed the exit.
Buck and the possums escape and go to Buck's hideout, where Buck explains how he used to be part of an old squad that established the watering hole for animals to peacefully co-exist. He goes on to say that Orson did not accept to join his squad, because he believed in a world where the strong dominate the weak with him being the leader of them all. Buck further explains that because his squad stood in Orson's way, Orson tried to get rid of them, but they defeated him, banishing him to exile on Lava Island. With no one knowing how he escaped, Orson figures out that he can control two raptors on the island with fire.
Later, the two raptors find Buck's hideout, but Zee, a zorilla who used to be part of Buck's former squad, saves them by using a gas to knock out the raptors. Orson, seeing that two raptors is not enough to capture Buck, goes to find more. Orson and his new army of raptors attack the watering holes with Buck and Zee telling the animals to get out of there. Buck and Zee, tense with each other after the squad broke up, head to get some help along with the possums. They arrive at Lost Lagoon and summon their old friend Momma T-rex. While Momma is knocked out because of a toothache they pull, Orson and his raptors arrive and attack them. Buck and Zee forgive each other for breaking up the old gang and agree to work together. Acting as a diversion, Buck is captured by Orson, allowing the others to escape.
Trying to figure out how Orson controls the raptors, Zee and the possums devise a plan to get Buck back. Ellie and the others figure out that Crash and Eddie entered the Lost World and uncovered the entrance. They run into Momma, who tells them Crash and Eddie are in danger. Zee and the possums free Buck, and with the help of the others, they continue to fight Orson and his army. Buck tries to explain to Orson that everyone needs to live in peace, but Orson arrogantly denies him and continues to fight. Figuring out Orson controls the raptors with fire, Crash and Eddie create a fire of their own and stop the raptors from fighting and save the Lost World. The raptors, mad at Orson, chase him away.
Ellie and the others apologize to Crash and Eddie and ask them to come home, but Crash and Eddie express that they want to stay in the Lost World. Sad, Ellie allows them to stay and they say goodbye, but Crash and Eddie come to visit them often.