The Black Mouth is a huge, unmanned fishing boat that stealthily prowls the Arctic Ocean, swallowing any animal in its path. Its dark presence spreads fear among the animals and the Inuit people.
When Lars, the friendly little polar bear, makes friends with Robby the seal and shares fish with the inhabitants to help them get along, the animals become worried when their food supply gets cut off.
Lars decides to go into the human settlement with Robby despite his friends' warnings that it is dangerous. He comes across Lena, an Inuit girl, who warns Lars about the Black Mouth, a giant metal monster devouring all the fish in the ocean and the culprit behind the food scarcity.
Lars rushes to warn the other animals even though his parents don't believe him, but it is too late. The Black Mouth breaks through the ice and captures all the animals, including Robby.
Lars decides to take down the Black Mouth by luring it towards a huge cliff. The evil robot gives chase and catches him, but Lars's friends Greta and Caruso arrive and set him free just before the Black Mouth runs aground on jagged rocks, freeing the animals.