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Reports of unexplained explosions of military craft emerge in England, and Linda Beech is assigned by Scotland Yard to work undercover to find the man responsible for the attacks. Newspaper reporter Derrick Holt is on his way to London to report on the coronation of King George VI, when he gets an urgent message for help from his friend Mary Smith in France. When he visits her, she tells him her husband Martin has disappeared, and among his photographs is a picture of Linda, who was headed to Montritz on the same airplane as Derrick. Derrick checks into the best hotel in Montritz, hoping to meet with Linda, and is an uninvited guest at a charity party hosted by Franz Jorg, of whom Linda and another agent, Lady Helen Lavering, are suspicious. During a musical performance at the garden party, Derrick sends Linda a note asking her what she knows about Martin, while Jorg orders one of his thugs to get rid of Derrick. Derrick receives an anonymous note telling him he can find Martin at a wax museum at an assigned time, and thinking the note is Linda's response, goes to the wax museum alone. Martin is fatally wounded, but before he dies, Derrick hears him whisper the word "exile." Derrick is soon captured by police, who hold him responsible for Martin's death because his gun was found at the scene. Derrick ably escapes the police, and upon his return to the party, exchanges clothing with a musician and hides in the orchestra, pretending to play the bass fiddle. The sour notes he plays draw attention to him, and the police chase him again until he meets with Linda, who pretends he is her drunken boyfriend. Back in the hotel, Linda reveals to Derrick that she, Helen and Bertie are all agents for Scotland Yard, and that they are suspicious of his activities. Derrick relates his involvement, and tells them of Martin's last word, which is a code to mean that Franz Jorg is the murderer. Helen is suddenly shot, and Derrick proves his trustworthiness by capturing the killer, whom Bertie then shoots. Linda later visits Jorg's estate, and he reveals that he is actually Winston Farnon, a brilliant inventor who was exiled from England for selling an invention to the enemy during wartime. He is now getting revenge against England by using his new invention, the death ray, and plans to use it during the King's coronation to destroy England's air fleet. After Jorg traps Linda in a room and escapes by airplane, Derrick and the police come to her aid. In spite of his fear of flying, Derrick goes up in a plane flown by Linda as her machine gunner to search for Jorg before he uses his destructive force. The coronation parade begins, and Jorg uses his death ray to destroy several planes that are flying in formation, before Linda and Derrick sight him. Derrick shoots out the death ray and riddles Jorg's plane with bullets, and it crashes to the ground, killing Jorg. Derrick and Linda finally approve of each other.