Yoon Ho and his colleagues arrive at the Leviathan, a nuclear power plant floating in the middle of the open ocean, for a routine safety inspection. After a mysterious explosion, they find themselves completely isolated with all outside communication severed.

The explosion is only the beginning of their nightmare as a mysterious giant emerges from the reactor smoke and begins hunting and devouring the survivors. Trapped on a sinking vessel, the survivors must wage a desperate struggle to stay alive against the man eating monster.