A group of fishermen in the small fishing town of Smallport kill a great white shark, only for it to immediately resurrect as a ghost inside a cave on the shoreline, and kill them. The next day, a group of friends consisting of one of the fishermen's daughters Ava Reid, her younger sister Cecily, and their friends Blaise Davis, Taylor, Cameron Stahl, and Mick, go swimming down by the sea. Ava, and Cecily see their father's abandoned boat, and swim to it, only to find a video camera, and their father's bloodied hat. After the group witnesses Taylor being eaten by the ghost shark, the beach is closed off, although the town sheriff Gary Martin refuses to believe the group that the shark was translucent. A lighthouse keeper named Darnell Finch intervenes, and tells the sheriff ghosts in the town, catching the attention of Ava, Blaise, Cecily, Cameron, and Mick. Cameron attempts to convince his father Frank, the mayor of the town, of the shark, although he refuses to believe him as well.The next day, Ava, Blaise, Cecily, Cameron, Mick, and Martin review the footage on the camera at the police station, where they discover that they had killed a shark. The teens theorize that the shark became a ghost, although Martin refuses to believe so. Ava, and Cecily attempt to enlist Finch's help, although to no avail. That night, Blaise, Cameron, and Mick go to a pool party. While Blaise leaves to check on Ava, and Cecily, Cameron, and Mick stay only to encounter the ghost shark again, and it kills two people, the second of which being Mick. After the ghost shark kills several more people in various forms of water, including pipes, buckets of water, and water slides, Ava, Blaise, and Cameron enter the lighthouse in order to find Finch, and he tells them of the cave, saying that it is connected to the ghost shark, although he falls asleep as a result of his drinking before he can reveal any more information.At Ava's house, Cecily takes a bath, only to be attacked by the shark, although the others arrive at the house in time to save her. Ava, Blaise, Cecily, and Cameron then read an article that says that Finch's wife Alison drowned in the cave during a high tide, leading to the possibility that she possessed the shark. They also realize that the shark can appear in any form of water, and can't manifest in anything outside of water. The group decides to head to a local museum to find more information about the lighthouse incident, only for a police officer to show up. While Cecily distracts him, Ava, Blaise, and Cameron head to the museum.At the museum, while Cameron stays behind for a smoke break, Ava, and Blaise discover from a museum worker that a Roanoke-like disappearance in the colony occurred when it was first colonized. A couple of diary entries from one of the colonists details that after a plague infected a few colonists, they were taken to the cave to be executed, and the cave's spirits killed all of the remaining colonists, with the last entry stating "whoever dies violently in the cave will rise again". The museum worker attempts to give Ava, and Blaise a book that helps ward off evil spirits, only to find that Finch has stolen it.Meanwhile, Cameron's smoking causes the museum's sprinkler system to activate, allowing the ghost shark to manifest. The shark kills the museum worker, and Cameron while Ava, and Blaise escape. Meanwhile, Martin, and Frank witness Frank's assistant being ripped in half from the inside out after he drinks some water. Frank later finds out about Cameron's death, and he, and Martin go out on the sea to kill the shark, although it quickly kills them instead. In the meantime, Ava, Blaise, and Cecily head to the lighthouse; on the way, Blaise, and Cecily crash their car after the ghost shark attacks them, and witness two punk kids being eaten by it.At the lighthouse, Finch is found with the shark's dead body in the cave, and he offers to help the group kill the shark. He says that the only way to kill the shark is by killing it with the weapon that originally killed it, in this case, a harpoon which had stuck to the shark's body. Ava successfully stabs the shark with the harpoon, although it is revealed to still be alive, and bites off one of Finch's fingers. When the group attempts to create another plan, Finch breaks down, refusing to kill the shark, and reveals that he had drunkenly murdered his wife in the cave during a fight.Ava, Blaise, and Cecily steal dynamite from a nearby explosives facility, hoping to blow up the cave with them, with Finch offering to help them after seeing a spiritual manifestation of his wife. They set up the explosives, and prepare to blow up the cave, although it begins to rain, preventing them from doing so. The shark then manifests, and eats Finch. Ava gets the fire in the cave going again, and escapes just as the explosives detonate, causing the ghost shark to finally disappear. Ava, Blaise, and Cecily begin swimming out into the distance as Blaise asks Ava on a date, to which she accepts.