The ship that the Tiger Shark torpedoed was actually a British medical ship (the same ship the British survivors were on), not a Nazi warship. The three other commanding officers killed Captain Winters to protect their careers when he wanted to immediately rescue the survivors and report the accident. Captain Winters' ghost kills the other two officers and Lieutenant Brice commits suicide out of shame. The remaining submarine survivors are saved by a British ship after they scuttle the Tiger Shark.
This story follows a US submarine named the USS Tiger Shark during World War II. Captain Brice (Bruce Greenwood) receives a message from his superiors ordering him to rescue some stranded sailors from a sunken British medical vessel. The submarine arrives and finds thethree survivors,Kingsley (Scott Foley), a nurse named Claire (Olivia Williams) and one of their patients. The sub needs to submerge quickly as a German warship approaches and Captain Brice orders complete silence. Suddenly, a radio turns on and the noise attracts the attention of the warship which drops depth charges. The sub manages to survive, though Brice is infuriated that someone turned on the radio. Brice discovers that the patient is German and shoots him in "self defense," believing the German betrayed their position. Kingsley and Claire are confined to their quarters, though they swear the German did not betray them.
Claire befriends a junior officer named Odell (Matthew Davis) and he tries to convince Brice to head towards a British seaport to release the survivors to the British. Brice refuses, saying they will sail back to the US. Meanwhile, the hydrogen levels in the sub begin to rise and strange noises are heard throughout the submarine. Some of the men, especially a crewman nicknamed Weird Wally (Zach Galifianakis) and his buddy Stumbo (Jason Flemyng) worry the sub may be haunted.
Claire eventually discovers that Brice was not the original captain of the sub and that the original commander was a man named Captain Winters (Nick Hobbs). Brice explains that the Tiger Shark attacked a German warship and then surfaced to confirm the wreckage. When they surfaced, they did not find the ship, but saw many German survivors and artifacts floating in the water. Brice and fellow officers Coors (Scott Foley) and Loomis (Holt McCallany) watch Captain Winters ignore the survivors' pleas and reach over the side to retrieve trophies from the water. Winters slips over the side, hits his head, and drowns. This leaves Brice in command.
As the journey continues, many tragedies befall the submarine. They are caught in a spiked submarine trap, the steering wheel breaks and prevents the sub from surfacing to vent the hydrogen and prevents them from changing course, and the crew becomes ever-more on edge. Oil begins leaking from an exterior breach and Coors, Odell, Wally and Stumbo are sent out in diving suits to patch the breach. Coors sees a phantom face in the hull and impales himself on an antenna while fleeing. Odell later tells Claire that he doesn't believe Brice's story about Captain Winters' death.
Claire confronts Brice and tells him she thinks the ship is haunted. He rudely dismisses her when a tragedy strikes. A fire breaks out and kills most of the crew. While investigating the accident, Loomis sees Captain Winters' ghostly face in a mirror and flees the submarine itself, dying as he attempts to swim deep underwater.
The ship finally arrives at its destination, the site where they sunk the Nazi warship. The submarine then begins to violently surface, killing Kingsley in the process. Wally notes there is a ship nearby (its nation of origin unclear) and tries to radio it for help, but Brice destroys the radio equipment. Claire deduces that the ship they attacked was actually the medical ship she and Kingsley were on, which looked similar to the German warship that Brice claims to have sunk. Brice tries unconvincingly to deny her accusation. Oday struggles with Brice for control of the ship while Claire sneaks on deck to try and signal the approaching ship with a flashlight. The rest of the crew prepares to scuttle the Tiger Shark while Brice pursues Claire.
On deck, Brice admits that he, Loomis and Coors killed Captain Winters in the heat of the moment. When the four of them realized the terrible mistake they had made in sinking the British medical ship, Winters attempted to try and immediately save the survivors. If he succeeded, their mistake would be exposed, their careers would be derailed and they faced jail time. Thus, the three killed Winters in a struggle to prevent him from saving the survivors.
The rest of the crew makes it to the deck, but see Brice holding a gun on Claire. He says that Winters didn't need to kill him like he killed Loomis and Coors, and then commits suicide with a bullet to the head. The remaining crew manages to signal the ship for rescue as the Tiger Shark sinks (the rescue ship turns out to be British).
Onboard the rescue ship, Oday admits that he has no idea what to tell his superiors. Claire agrees that there is no good story to tell, but theorizes that the ghost of Winters wanted the Tiger Shark to go down at this location for a reason. The camera then follows the descent of the Tiger Shark as it hits the ocean floor next to the wreckage of the British medical ship it accidentally destroyed - the two ships finally reunited per Winters' wishes
Thanks Evan B!