Mystery Submarine is a 1950 American war film directed by Douglas Sirk and starring Macdonald Carey, Märta Torén and Robert Douglas. Plot Madeline Brenner, a German naturalized as an American, is being questioned by the United States Attorney for the Southern District of New York, who is considering treason charges against her. In flashback, she describes how she worked on Cape Cod as a secretary for a wealthy woman, Mrs. Weber. Madeline believes her husband died five years earlier, in 1945, when his U-boat, the U-64, was sunk. She is approached by a mysterious man on the beach, who says her husband is still alive. He says she can be reunited with him if she helps kidnap Dr. Gurnitz, a noted German scientist visiting Mrs. Weber. During a yacht cruise the next day, Madeline disables the radio as the U-64 surfaces. The submarine’s crew kidnaps Gurnitz. Onboard the U-64, Madeline discovers she has been lied to. Her husband is dead, although his submarine survived. The man from the beach, Eric von Molter, is now in command, and used her to get Gurnitz. Von Molter sinks the yacht, killing Mrs. Weber and the passengers and crew. Captain Elliott and Doctor Brett Young next give testimony. Elliot describes how the Navy figured out the yacht was sunk by a German torpedo and learned the renegade submarine might be operating out of a secret base near the fictional town of Belgrana in an unnamed South American country. They sent Young, one of their best spies, to investigate. Young describes how he searched the coast, discovering the cove where the U-64 was hiding. Young is caught, but von Molter buys Young’s cover story, that he is a German submariner who escaped an American POW camp. Von Molter invites Young to be their medic, tending to Gurnitz. Madeline feels she’s a prisoner on von Molter’s estate, but he is determined to win her heart. Von Molter plans to trade Gurnitz on the high seas in a few days’ time. Young attempts to use the U-64’s radio to warn his superiors, but von Molter catches him before he can send a message. Madeline attempts to escape the estate, but Young stops her, saying she will never get out alive. Young tells von Molter he needs medication for Gurnitz and wants to go to Belgrana on his own for it, hoping to radio for help, but von