In August 1943, the U.S. Navy submarine USS Tiger Shark roams the Atlantic during World War II, executing a cautious patrol that could tilt the balance of small, high-stakes encounters at sea. When a British PBY Catalina spots survivors adrift, Tiger Shark is ordered to rescue them from the recently sunk hospital ship Fort James. Among the rescued is Claire Paige, a British nurse, and two men, one of whom is wounded. One survivor insists he saw a German U-boat on the surface just before Fort James took a torpedo hit, raising the tension aboard as the crew processes the fragile line between luck and peril.
At close range, danger returns in the form of a German destroyer closing in on the rescue scene. Tiger Shark endures several tense confrontations, taking damage from depth charges that test the submarine’s structural integrity and the crew’s resolve. The wounded survivor is later revealed to be Bernhard Schillings, a German prisoner of war. Lieutenant Brice, the submarine’s commanding officer, suspects that Schillings has been used to betray Tiger Shark’s position to the pursuing warship. The accusation becomes brutal when Brice confronts Schillings, and, in a moment of panic, Schillings grasps a scalpel in self-defense and is killed by Brice.
Brice later confides in Paige that Tiger Shark has just since sunk a German submarine tender, and that the prior commanding officer, Lieutenant Commander Winters, died after surfacing to confirm the sinking. Brice explains that Winters attempted to salvage a boathook from the debris, an action that led to a fateful accident when the submarine struck a submerged object, causing Winters to fall overboard and drown. With Winters gone, Brice claims the command role, accepting the heavy burden of responsibility for what happened.
Soon after Schillings’ death, eerie phenomena begin to plague the crew. Disembodied voices and other unsettling disturbances unsettle the ship, casting doubt on Brice’s version of events. Ensign Douglas Odell questions whether the ballast-tank discussion about Winters’ death is accurate, sensing a mismatch between the official account and what he feels beneath the surface. Lieutenant Steven Coors counters with the possibility that Winters’ death involved a more complex series of actions, and he urges Odell not to reveal the truth. Coors himself dies in a mysterious accident as the narrative begins to shift from rumor to possible confession.
As Tiger Shark endures a cascade of strange mechanical failures, the crew suspects the submarine is haunted, its engines and systems seemingly acting of their own accord. The ship lurches back toward the site of the previous sinking, almost as if drawn by an unseen force. Loomis, another officer, reports seeing Winters’ ghost, and desperation drives him to attempt an escape via an outside trunk while submerged; he is killed when he is impaled on an external railing. A crewman nicknamed “Weird” Wally concludes that a malediction haunts the vessel and insists that it must be satisfied to sever the netherworld’s hold.
Paige and Odell uncover a heartbreaking truth: Tiger Shark mistook Fort James for the German submarine tender, and in sinking the British ship they have compounded a grievous error. The pair also discovers that Brice, Loomis, and Coors believed they could not afford this major blunder to appear on their records, so they conspired to suppress the truth, effectively silencing Winters on the ship’s deck as he tried to help the Fort James’ survivors. The submarine continues to suffer, and only five survive: Brice, Odell, Paige, Stumbo, and Wally.
When Tiger Shark finally locates the wreck site of Fort James, the vessel surfaces in a crippled state, lacking the capability to escape. The crew detects a surface ship close by, and Paige defiantly signals with a flashlight. Brice confronts her, holding her at gunpoint while wrestling with his conscience. In a moment of profound remorse, Brice admits the full cover-up and, overwhelmed by guilt, shoots himself and falls into the ocean’s depths.
The surface ship that approaches is British, and it rescues the four remaining survivors. With the escape route closed and their options dwindling, Tiger Shark sinks to rest on the ocean floor, lying beside the wreck of Fort James as the truth of what happened remains buried beneath the waves.
