Survivors escape to a deserted atoll, after their boat during a Semester at Sea ship is sunk by a mutated two-headed shark. But when the atoll starts flooding, no one is safe from the double jaws of the monster.

In search of a new energy source, Russia accidentally reawakens the Kolossus – a giant robot doomsday device from the Cold War. At the same time, a new Mega Shark appears, threatening global security.

As a mutated, three-headed, great white shark eats its way from one end of the ship to the next, the passengers have to fight the deadly predator using anything they can find.