When Jim's big brother Jack is released from prison, the brothers—along with their broken father and Jack's menacing best friend—decide to charter an ocean fishing boat to celebrate Jack's new freedom. Once the small crew is far out to sea, however, a mutant species rises from the deep abyssal darkness to terrorize the vessel and its occupants. As the horror of their situation becomes clear, the small group must find a way to fend off the attack and somehow, someway, return to safety; but as the strange parasitic creatures overrun them, they must use more extreme—and deadly—measures to survive.