Drakma, an international spy, seeks the possession of the triangulator (a device emitting electronic death rays), which radio expert Alan Holt invented for the United States Government. Mary Walsworth manages to smash the machine before Drakma can get it; therefore, the villain captures Alan and Mary, puts Mary on a rum-runner, and leaves Alan on an island to build another triangulator--under threat of Mary's death. Instead, Alan builds a radio and broadcasts an SOS, which is answered by Admiral Walsworth, Mary's father. Alan gets to the rum-runner on a raft, and holds off the crew until sailors arrive to the rescue.