Jennifer Remming (Jenn) goes on an ocean excursion with her boyfriend Lucas Griffin, Mia Reed, and strangers Brad and Zach. After their boat is destroyed during a storm, Jenn washes ashore a deserted island.Brad also ends up on the island, but dies from his wounds. Jenn buries Brad's body only to find it dug up and missing the following morning.While swimming, Jenn discovers a bottomless black hole in the ocean floor not far from the beach.Jenn fires a flare when a plane passes overhead during the night. Jenn fires too late for the plane to spot her, although she illuminates a frightening creature standing in the sea.Jenn sleeps inside a hollow log for protection. The creature attacks Jenn's log, but returns to the water at daybreak.Jenn uses a small shark caught by spear-fishing to bait the creature the next night. However, the creature merely tears the meat from the rope and disappears again.Zach's dead body washes ashore bearing unusual knife wounds. Jenn uses the body to bait the creature, but it escapes again.Jenn builds a suspended hammock so she can sleep out of the creature's reach. The creature still pulls Jenn down, but she manages to hide until the creature retreats at dawn.After a full week passes, Lucas and Mia arrive on the island in a raft. Jenn warns Lucas and Mia that there is an amphibious monster that drags its prey back to a hole in the ocean, but they do not believe her. Jenn proposes escaping the island in the raft, but Lucas and Mia furtively refuse to leave.Jenn finds blood on Lucas' pocketknife. Jenn also becomes suspicious of Lucas and Mia's relationship. Jenn tries fleeing in the raft alone, but Mia knocks Jenn unconscious and Lucas ties her to a tree.When Jenn recovers, Lucas confronts her about their troubled relationship. Jenn asks Lucas what happened to Zach, but they are interrupted when the creature attacks Mia and pulls her into the ocean.The next day, Jenn and Lucas pack the raft with supplies and set out to sea. Jenn notices that the inside of the raft is covered in Zach's blood.The creature attacks the raft and pulls Jenn underwater. Jenn fights back using the pocketknife. The creature attacks Lucas and pulls him into its hole instead.Jenn returns to the island. During daylight, Jenn prepares kindling and makeshift spears for battling the creature. The creature swims ashore at nightfall for a fiery final confrontation. Jenn ultimately kills the monster.