In Shanghai, American soldiers of fortune Tom Baldwin and Joe Dugan are hired by a Chinese general to deliver the priceless Soo diamond to a storekeeper named Chang. Moments after Tom and Joe hand the diamond to Chang, his shop is attacked by soldiers, and during the ensuing chaos, the diamond is stolen. Desperate to retrieve the jewel, Tom and Joe corner everyone in the vicinity and question them about their activities. Included in the group of suspects are Jane Dunn, a pretty American; Anthony Durand, an American mystery writer; Durand's British valet, Harvey Dinwaddle; an alcoholic named Philip Burton; silly Mrs. Katharine "Katie" Collins; and the mysterious Dr. Feng Tu. Unable to locate the diamond, Tom and Joe release their suspects but trail Jane, who meets secretly with a Chinese man claiming to know the identity of the thief. Before the informant can reveal his information to Jane, however, he is killed by an unseen assailant. The next day, Joe and Tom track Jane to the docks, where the San Francisco-bound S.S. Asiatic is about to sail. To Tom and Joe's surprise, all of their suspects board the Asiatic . Soon after sailing, Jane receives an on-board telephone call from an unidentified man who warns her not to drink her morning coffee and advises her to get off the ship in Yokahama. Later Jane discovers that a box of chocolates she had received as a gift is laced with poison. Next, Durand and Harvey's cabin is ransacked, and Jane's room is burglarized. Furious, Jane shows the ship's captain a letter from the United States Customs Office, which identifies her as a special agent, and demands to search Tom and Joe's cabin. There Jane and the captain discover Joe's dead body. After learning that Joe had borrowed a book about the human stomach from the ship's hospital, Jane sneaks into Tom's cabin and is knocked unconscious. Tom then enters and struggles in the dark with Jane's attacker. After the tussle, Tom finds a Buddha charm on his floor, which he knows belongs to Katie. Jane, who has fallen in love with Tom, confides her identity to him, and together they approach Katie. By the time they reach Katie's cabin, however, they discover that Katie, who is actually an undercover insurance investigator, has been poisoned. Using fingerprints found on Katie's fatal cocktail glass as a comparison, Jane checks all of the remaining suspects' fingerprints and identifies Tom as the killer. Later, however, Jane reveals to Tom, who is locked in the ship's brig, that the killer had framed him by substituting Katie's glass for one used by him. While Jane questions Durand about Katie, a still imprisoned Tom is shot and wounded. Then, shortly before landing in San Francisco, Jane also is shot by an unseen assailant. At the dock, Harvey is accused of smuggling jade and is searched by customs officers. After Jane reveals that her shooting was a diversionary fake, she accuses Harvey of stealing the diamond. Durand then rushes in with a gun and turns off the customs office's lights. During the ensuing fight, Dr. Tu, actually a representative of the general, shoots Durand. Once the murderers are revealed, Jane accepts Tom's marriage proposal and, while the captain of the Asiatic performs their wedding, explains how she solved the crime: After reading a radiogram sent to Katie just before her death, which identified one of the thieves as a former circus performer, Jane arranged for Harvey, whom she suspected was the circus "freak," to be X-rayed in the customs office. As predicted, Harvey's X-ray revealed that had he swallowed the diamond.