In Central Park Zoo, Samson the Lion tells his 11-year-old son Ryan stories of his adventures in the wild. That night, when the zoo closes, all the animals are free to roam. Samson, along with his friends Benny, Bridget, Larry, and Nigel compete in a turtle curling championship while Ryan is with his friends. Ryan accidentally causes a stampede which head to the game. Samson and Ryan have a falling-out and Ryan storms off before Samson can apologize. Ryan wanders into a green box and gets shipped away by a truck, which the legend tells will take him to the wild. With the help of a pigeon, Hamir, Samson and Benny go after him, sneaking into a garbage disposal truck with Nigel, Larry, and Bridget coming as well, accidentally throwing Benny off the truck. After passing through Times Square and nearly being crushed in the garbage disposal, the group encounters a pack of rabid stray dogs. Samson leads them through the sewer rather than scaring them off. There, they take directions to the docks from two street wise alligator brothers, Stan and Carmine. The next morning, the four friends steal a tugboat during a hectic escape from New York harbor. With help from Larry, they drive the boat and reunite with Benny, who has enlisted a flock of Canada geese to help lead the crew in the right direction of Ryan’s ship. Days later, Nigel goes mad and, under the impression they have hit an iceberg, jumps overboard.
The boat runs aground in Africa, where the group quickly discover that all the animals in the area are being evacuated by the carriers, as a nearby volcano erupts. They witness Ryan escaping, but he runs into the jungle. Samson attempts to find him, but it is quickly revealed that he has in fact never been in the wild before after trying to eat a rude Hyrax. The rest of the group head back to the ship, but Samson decides to keep trying to find his son. While walking, Samson starts seeing plants and rocks change colors. Nigel is captured by a group of wildebeests who dwell in the volcano, and their leader Kazar, pronounces him ‘The Great Him’, based on an ‘omen’ he received when he was young: about to be devoured by lions, a toy koala fell from the sky (actually from a plane) and scared the lions away, saving his life. Kazar wants to change the food chain; he would rather see “prey become predators” and vice versa. For this, he needs to eat a lion. Bridget and Larry are also captured and held prisoner.
Ryan hides up an old tree, but is attacked by a gang of vultures led by Scraw and Scab acting under orders from Kazar. The branch breaks and traps his paw. Samson hears Ryan’s cries and runs to save him, scaring off the birds. The two reunite, but are interrupted by a herd of wildebeests. Ryan is shocked when Samson tells him to run. The two retreat to a tree where Samson reveals the truth. He was born in the circus and like Ryan, was unable to roar. Samson’s disappointed and wicked father claimed that if he had been born in the wild, he would’ve known how to roar before allowing him to be sent to the zoo, where he became an orphan and lied to avoid the shame. The wildebeests discover them and, in the scuffle, send the tree over the cliff, with Samson still hanging on. Ryan is captured and taken to the volcano.
Benny finds Samson and encourages him to be himself, even if he’s not from the wild. They find two chameleons, who were leading Samson to the volcano and are also trying to defeat Kazar’s army of his kind. Samson uses the chameleons’ camouflage abilities to slip into Kazar’s lair. Nigel tries his best to stall the wildebeests from cooking his friends, and eventually Samson fights Kazar but is overwhelmed. Ryan, seeing Samson in danger, climbs onto a catapulting device and launches himself at Kazar, finally letting out a roar. With Kazar distracted, Samson manages to defeat him. Ryan tells Samson that he is happy to have him for a dad. The other wildebeests are touched by this and refuse to serve Kazar any further. Samson gains the courage he has needed and roars powerfully enough to push back a charging Kazar. The animals, along with the wildebeests flee except Kazar, who is trapped in the erupting volcano. The animals manage to escape on the boat and travel back to the New York Zoo.