Writer/director Rob Grant wastes zero time getting us in the right frame of mind with Brett Gelman’s perfectly sardonic voice narrating an auspiciously violent collision of three close friends caught in a lovers’ misunderstanding. Harpoon‘s first scene provides effect—a stranded yacht in the middle of the Atlantic Ocean with “Sos” duct taped to the deck—while the second supplies cause as Jonah (Munro Chambers) gets his face punched in by best friend Richard (Christopher Gray) with the latter’s long-term partner Sasha (Emily Tyra) imploring him to stop. Move those temperamental seeds of mistrust to a boat’s tight quarters and we can guess why the vessel will eventually find itself in distress. The question then becomes which, if any, of those passengers will be alive to accept help.

The misunderstanding: Richard thinks Jonah and Sasha are having an affair. It stems from texts found on...