In a marriage boot camp on an island, William Nix (Brandon Auret) is counseling a group of couples through various physical challenges to make them more cooperative with each other. William's wife Suzanne (Caitlin Harty) arrives on a boat so he can sign some divorce papers, leaving the couples to swim to a small dock in the water as part of a challenge. Meanwhile, William's assistants Brad and Rebecca are eaten by a 6-headed shark while swimming in the water. William goes back onto the island to meet up with the couples, leaving Suzanne on her boat with her captain. On his way, William discovers Brad's severed foot, and goes to warn the couples.The couples, consisting of Mary and James, Daphne and Duke, Jake and Sarah, and Kip and Angie, swim out to the dock, only to discover the severed foot. William arrives immediately after, and the group uses the dock to get to a the Sweet Waters, a boat that Brad and Rebecca came in on. They fail send out the mayday signal, and William, and Duke swim back onto the island in order to find Rebecca, not knowing that she's dead. William and Duke are soon attacked by the shark, although Duke seemingly kills it. However, the shark soon revives and eats Duke. When Williams swims back to the boat it runs aground, preventing it from going anywhere.With no other options left, the survivors swim back to the island, only to discover that a storm's approaching the island. Knowing that the storm will flood the island, the survivors swims to an abandoned research facility on a series of floating docks next to the island, only to discover blood all over the walls in a room with a small fishing hole in the middle of it, indicating an attack took place there. William attempts to contact help with a radio in the facility, although to no avail. The shark soon enters through the hole and eats Sarah, and Jake. William has the others escape while he stays behind to fight the shark.With the others safe at shore, William manages to contact Suzanne through the radio, and explains their situation to her. She agrees to rescue them at a nearby lighthouse on the island. As William begins to swim back to the island, the shark begins walking on the island using four of its heads as leverage, and kills and eats both Kip and Angie. William arrives with a machete and decapitates one of the shark's heads, causing it to back off, although it later grows back the decapitated head. The remaining four survivors, William, Mary, James and Daphne, reach the lighthouse. A delirious Daphne walks off onto the shoreline alone, where the shark eats her.The shark attacks Suzanne's boat, and eats the captain. Suzanne attempts to swim to shore but she too gets eaten, devastating William. William, Mary, and James then make an attempt to kill the shark by setting up several air tanks on the beach, with James shooting them from the top of the lighthouse when the shark arrives. The shark soon arrives, and James shoots, and kills one of the heads, only for one of the other heads to rip off the dead head, and use it as a projectile to kill James. The shark retreats back into the water while William and Mary find that the Sweet Waters is now free from the rocks.William and Mary set up the air tanks on the Sweet Waters, hoping to blow it up, and kill the shark. However, although William manages to kill one of the shark heads, the plan fails, and he and Mary are forced to swim back onto the island with the boat sinking. The shark, which has all six of its heads back, follows them, although William lures it next to some of the tanks left on the shore line, and shoots them with a flare gun, killing the shark. William and Mary, the only survivors of the massacre, begin walking to safety as the decapitated shark heads attempt to bite each other.