Determined to "go straight" and marry his attractive helpmate, Helen Heming, English jewel thief Oliver Lane, who is known in Europe as the "Solitaire Man," is dismayed when Robert Bascom, his drug-addicted cohort, presents him with a necklace stolen from the British embassy in Paris. After he convinces Robert, who also loves Helen, to give up the necklace, Oliver goes to the embassy late one night and, breaking into a safe, places the jewels inside. As soon as Oliver sneaks away, however, another man sneaks in and steals the necklace from the safe. Before the man escapes, Inspector Kenyon from Scotland Yard bursts into the room and is shot and killed by the robber. In the chaos, Oliver retrives the necklace and part of the killer's watch chain. With Helen and her mother, Mrs. Vail, who poses as a poor British aristocrat in order to sell Oliver's stolen jewels to unsuspecting Americans, Oliver heads for the Paris airport. Oliver, Helen, Mrs. Vail and Robert are joined on the Paris-to-London airplane by another man and Mrs. Hopkins, a garrulous, wealthy American on vacation with her husband Elmer. As the airplane is taking off, Mrs. Hopkins demands that the pilot stop to pick up her husband, who has arrived late at the airport. Although Oliver also is anxious to stop, the pilot is reluctant, and the other man insists that they continue. Once airborne, the man, who identifies himself as Inspector Wallace of Scotland Yard, accuses Oliver of being the Solitaire Man and demands, at gunpoint, to inspect his luggage. While Oliver calmly denies Wallace's charges, Helen and the others overpower Wallace, take his gun and handcuff him. After the group becomes aware that another plane, which Wallace claims is an official French army transport, is following them, they debate how to handle the inspector. Convinced that Wallace has the upper hand, Oliver offers to give himself up and turn over the necklace in exchange for Helen, Robert and Mrs. Vail's freedom. Although Wallace shows interest in the offer, Oliver turns off the plane's interior lights to throw off the pursuing French plane. When Oliver then questions Wallace about his presence on the flight, Wallace reveals that Robert had tipped him off about his identity. While Robert admits that he had talked with Wallace and contemplated informing on Oliver to collect a 10,000 pound reward, he denies carrying out his plan. After Robert unsuccessfully accuses Mrs. Vail of the betrayal, Wallace reminds Oliver of the evidence against him, including Robert's confession. Overwhelmed with guilt, Robert jumps from the plane to his death rather than testify against his friend. Helen then notices that Wallace is carrying a pocket watch with a broken chain, and Oliver accuses Wallace of being both a police informant and Kenyon's killer. Unable to land the plane prematurely because of fog, Oliver gives Wallace the necklace and his now empty gun with the understanding that Helen and Mrs. Vail are to be left alone. When the group finally lands in England, Inspector Harris and other policemen greet them, and a confusion of identities ensues. After Oliver insists that he is Wallace, and that Wallace is the Solitaire Man, he reveals to Harris that it was he who had signalled to the French plane by turning out the other plane's lights. Oliver then tricks Wallace into revealing himself to Harris as the jewel thief and Kenyon's killer. While trying to escape, Wallace is shot by Harris, who then accepts Mrs. Hopkins' story that Oliver is a legitimate jewelry dealer who was appraising the necklace that Wallace had tried to sell her on the plane. Thus cleared, Oliver and Helen head off to their new lives as a quiet, happily married Devonshire couple.