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Alan Holt (Antonio Moreno) is a radio expert who has invented a death ray machine for the U.S. government. International spy Drakma (Tyrone Power Sr.) wants to get his hands on the invention and he sends his henchmen to attack Holt in his laboratory. Holt's sweetheart, Mary Walsworth (Agnes Ayers), is there with him and she smashes the death ray. She and Holt are captured and taken on Drakma's yacht. The spy puts Mary on a rum-runner and Holt in a workshop on a lonely island. To save Mary, Holt is ordered to build another death ray. He agrees, but instead he builds a telegraph machine and calls for help. Mary's father, the admiral of a battleship, receives Holt's message and comes to the rescue. He sends a plane to sink Drakma's yacht, and Holt takes a raft out to the rum-runner, where he holds off the crew until the arrival of Walsworth's ship.