When a number of their aircraft mysteriously crash or disappear, the Goering-Gage Aviation Corporation comes under the suspicious eye of Inspector Gallagher of the United States Department of Commerce. Gallagher visits Goering-Gage and brings with him test pilot Jerry Blackwood of the United States Army Reserve. Jerry takes some of the Goering-Gage planes on test flights, and finds nothing wrong with them. When a dying passenger from one of their crashed passenger plane claims that they were attacked by a mystery plane, Henry Goering hires a psychiatrist, Dr. Norris, to question the man. Dr. Norris tells Goering that he believes a psychotic ex-World War I flying ace, whom he dubs "Pilot X," may be behind the assaults. With the help of Blackwood, Goering and Norris assemble a group of five ex-flying aces living in the area; German Lieutenant Baron von Guttard, French Lieutenant Rene Le Rue, British Captain Roland Saunders, Canadian Lieutenant Douglas Thompson, and American Lieutenant John Ives. The five aces are asked to assist Blackwood in the capture of Pilot X. All agree, and von Guttard comes under immediate suspicion when he violently argues with Le Rue, then acts ill at ease around Goering's son Carl, an ex-German prisoner of war. The next day, the pilots begin their patrols and Pilot X attacks, killing von Guttard. Later that day, Le Rue receives a telegram calling him away, and he borrows a plane. That night, Ives insists on going into town on business, and Le Rue is killed by Pilot X before Ives returns. The next day, Saunders has a mental breakdown, claiming that they are all "killers" for their past actions in the war. Blackwood receives a note from Pilot X, asking him to meet him in the sky at six o'clock the next morning. Thompson, meanwhile, receives a similar note, and Pilot X paints an "X" on Thompson's plane. That morning, Blackwood mistakes Thompson for Pilot X, and shoots down the Canadian, killing him. When a paint can is found in Ives' locker, all accuse the American ace of being Pilot X. Blackwood then makes an open challenge to Pilot X to meet him alone the next morning. That night, Dr. Norris calls the elder Goering, telling him that he knows the identity of Pilot X, but the doctor is murdered before he can divulge the murderer's name. Gallagher arrives at the airfield the next day, proclaiming that Blackwood is Pilot X, and sends Ives and Saunders after him. Meanwhile, Helen Gage, Henry's beautiful ward, first finds part of Saunders' goggles near Norris' dead body, then finds the other half in the Englishman's plane. Saunders, still half-mad, sees Helen in his plane, and takes off after Blackwood with her aboard. As Blackwood patrols the skies, Pilot X attacks Saunders, wounding him. Pilot X then turns his attentions to Blackwood, and they partake in a fierce dogfight. Blackwood shoots down Pilot X, who is discovered to be Carl Goering. Blackwood later finds a photograph of Carl in a German uniform with von Guttard among the wreckage of Carl's plane. It is discovered that Carl was not a prisoner of war, but deserted and joined the German air force under the name Muller, and that Norris had left a note proclaiming Carl to be Pilot X. With the mystery solved, Blackwood and Helen discover their attraction for each other as planes pass overhead.