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During World War II, American radio scientist Bergstrom is murdered by German agents, who want to obtain his newly developed radio transmitter. Soon American oil tankers are being sunk by enemy submarines, and Naval Intelligence has proof that an illegal transmitter has been relaying shipping information from an unidentified location in a Pacific coast city. Try as they might, government agents are unable to locate the transmitter. Assuming that whoever is using the transmitter will eventually need a technician to maintain it, FBI agent G. B. Fleming develops a plan in which he has all the local radio technicians fired from their jobs. Engineer Lewis J. Deerhold is furious about losing his job, and suspects that he was fired because he is not a citizen of the United States. Now that he is jobless, Lew worries about his young ward, niece Tina, a war orphan who needs a brain operation. After several rejections, Lew applies for work at a radio repair shop, and after leaving the shop, he encounters Ann Patterson, whose purse has been snatched. Lew recovers the purse and invites Ann to dinner. When he returns to his apartment, the radio shop owner is there with Dr. Arthur Huneker, who runs a hot springs resort, and his German interpreter, Vincent Bela. Huneker hires Lew to work on a therapeutic machine that employs radio principles, and gives Lew a cash advance. When another ship from the Bambridge Shipping Company is sunk, FBI officials arrest Bambridge's trusted guard, but he commits suicide before they are able to interrogate him. Huneker, who is actually a Nazi spy commanding the destruction of the tankers, sets Lew to work on repairing the transmitter. One evening, Ann, an FBI agent, steals the blueprints she finds in Lew's office. Because the initials on the blueprints are those of Bergstrom, Lew is questioned by the FBI, and he soon discovers that his meeting with Ann was a set-up. When he returns home, FBI agent Freddie Grayson is found dying from a gunshot wound, and tells Lew that his "Nazi doctor friend took the transmitter" and shot him. After approving Tina's operation, Lew escapes from the police and goes to the hot springs to confront Huneker. Huneker, who is consorting with Captain Hargas and Japanese commander Toyo, invites him to join the Nazi party. Lew pretends to accept until he finds them torturing Bambridge for shipping information. Lew is ordered to kill Bambridge, but instead he attempts to escape. After Bambridge is shot, Lew pretends that he has been shot and killed, then sneaks back into the hot springs and cracks the scrambler codes. Lew then escapes from the hot springs and encounters Ann, who has been investigating the resort. They go to Bambridge's shipping office, and are surprised to find Bambridge apparently alive. Bambridge, who is actually Captain Hargas in disguise, agrees to go to the FBI offices with them, but instead, takes them back to the hot springs where Huneker locks them in a steam room, which is going full force. Desperate to escape before the pressure kills them, Lew uses his ingenuity and creates a transmitter from electrical wires, and sends an S.O.S. message over public radio waves. A little boy named Johnny, listening on his short-wave radio, receives the message and alerts his father, who calls the FBI. After getting the call, Fleming alerts the bomber squadron, which flies over the coast and bombs the Japanese submarine lurking in the water. Johnny then accompanies the FBI agents when they rush to the hot springs to save Lew and Ann, and Huneker and his cohorts are arrested. Tina recovers from her operation, and as a reward for serving his country so well, Lew receives his citizenship and is inducted into the Army.