Based on the play by Bella and Samuel Spewack, a famous jewelry thief, wishing to retire, is drawn into another case and outsmarts a murderer on an airplane.
Mrs. Vail (May Robson) sells a necklace as "Nell Gwyn's" to an American couple for 5,000 pounds in France. Bascom (Ralph Forbes) kisses Helen (Elizabeth Allan); but she is in love with Oliver Lane (Herbert Marshall) and is happy when he asks her to marry him. He has bought a house in Devonshire. They plan to retire, but Bascom pulls a gun on Oliver, who talks him into giving it up. Bascom has stolen the Brewster necklace from the embassy. Oliver, worried about being caught as the "solitaire man," plans to return it. In the dark an inspector is killed trying to arrest a man opening the safe. Oliver tells Helen he didn't do it, but he was there and kept the necklace. She says she won't marry him unless they are honest. He wants to mail it back, but she says there isn't time before their flight to London.
On the plane with the jewelry gang of Oliver, Helen, Mrs. Vail, and Bascom are the vivacious Mrs. Hopkins (Mary Boland) and an inspector Wallace (Lionel Atwill), who accuses Oliver of being the solitaire man, asks for the necklace, and arrests him for murder. Helen helps Oliver get Wallace's gun, and Bascom handcuffs Wallace. Oliver says Wallace will "commit suicide" when they throw him from the plane, Mrs. Hopkins siding with the gang of thieves. Or they might get the plane to land; but a French army plane is following them. Oliver will give himself up to Wallace if the others can go free. Then Oliver learns that Wallace was informed by a letter from Bascom. Oliver says Wallace is a crook and a police informer. The desperate Bascom jumps out of the plane. Oliver accuses Wallace of murdering the inspector and turns out the lights. Oliver agrees to confess to everything if the others can go free, and he gives Wallace the necklace and the unloaded gun. When the plane lands, a man from Scotland yard questions everyone, accuses Wallace, and shoots him when he pulls the unloaded gun, finding the necklace in his pocket. He thanks Oliver for radioing from the plane. He and Oliver don't think the solitaire man will ever pop up again, as Oliver, Helen, and Mrs. Vail go off together.
Justifying their jewelry dealings as a way to help his dead war buddy's relatives and because he couldn't find a job, Oliver is ready to retire from the sordid business. He cleverly solves the case begun by Bascom's blundering theft in a plot with many surprising twists.