Major Charles Hammond of the British secret service suspects that the disappearance of a number of test planes carrying secret war department apparati is more than merely coincidence, but his suspicions are met with skepticism on the part of the aircraft manufacturers and the secret service. Meanwhile, at the Barrett and Ward aviation factory, another flight is scheduled for takeoff, despite the warnings of test pilot Tommy McVane, who also suspects sabotage. As the flight takes off and soars over the sea, a radio beam from the salvage ship Viking disables the craft's engines and sends it crashing into the water. The German agents aboard the Viking then seize the plane, only to find that the secret apparatus is not on board, having been removed by Hammond before takeoff. McVane, outraged at the disappearance of the plane, blames Barrett for failing to conduct a thorough investigation, while Hammond's sister Kay, a newspaper reporter, follows the story by going undercover as a waitress at the plant. Back at the Viking , the Baron, the ringleader of the spies, is enraged because they have failed to obtain the secret device, and orders the elimination of Jenkins, the traitor at the Barrett plant who had been selling them information. Jenkin's death brings together Hammond and McVane, who both suspect sabotage, but their investigation is cut short by the clever Baron, who instructs his men to let the test craft wash ashore, thus making the disappearance look like an accident. The Baron's plan works, and as Hammond is removed from the case, McVane is ordered to pilot the next flight. Hammond refuses to quit however, and traces all the disappearances to the Viking , just as the ship has McVane's plane in its sights. After he is captured and imprisoned with the fliers from previous flights, McVane incites the pilots to break out of the hold and battle the crew of the Viking . Meanwhile, Hammond commandeers a destroyer and sails to the Viking just in time to insure a British victory.