It's August 1943 and Lieutenant Brice (BRUCE GREENWOOD) and his crew of the U.S.S. Tiger Shark have rescued three survivors from a hospital ship torpedoed two days earlier. They are navigator Kingsley (DEXTER FLETCHER), British nurse Claire Paige (OLIVIA WILLIAMS) and her badly wounded patient, Schillings (JONATHAN HARTMAN).
With England being a 300-mile detour off their current course, Brice informs the survivors that they have no choice but travel with them and the sub quickly dives below the surface to avoid detection by an enemy ship. During a close encounter with some depth charges, a record player suddenly starts playing and gives away their location, eventually leading Brice to kill Schillings upon learning that he's German and believing that he tried to sabotage them.
Not long after that, various people, such as Claire as well as crewmembers Hoag (ANDREW HOWARD) and Stumbo (JASON FLEMYNG), begin experiencing weird and spooky events on the sub. Ensign Odell (MATT DAVIS) can't believe that anyone on board the sub would purposefully try to sabotage them, but second-in-charge Lieutenant Loomis (HOLT McCALLANY) quickly dismisses any sort of jinx or supernatural explanations.
After Claire starts poking around and noticing odd and missing entries in the captain's journal, Brice informs her of the accident that took the life of the former commanding officer, Captain Winters (NICK HOBBS). Yet Claire doesn't believe him, and soon other crewmembers such as Coors (SCOTT FOLEY), Chief (NICK CHINLUND) and Weird Wally (ZACH GALIFIANAKIS) begin to notice both the weird occurrences and Brice's increasingly volatile nature.
As the sub tries to return to its Connecticut base, more bizarre, seemingly supernatural and deadly occurrences ensue, all of which endanger the sub and its remaining crew as they try to figure out what's going on.