Movie star Philip Trent, tired of playing "Selby James the Super Sleuth" and thinking that the Peter Dean novels featuring the detetcive are unbelieveable trash, decides to take a cruise, much to the chagrin of his studio. His vacation starts off badly when he is mobbed by fans at the dock and asked for autographs on board. Then, in his cabin, Philip is approached by a beautiful blonde claiming to be pursued by a man with a gun. While he fixes her a drink, she sneaks out of his cabin with a letter opener. He looks for her in the fog, but returning to his cabin, he finds a body, dripping with blood. After summoning ship's captain Bromley and a detective, Sergeant Donovan, Philip returns to the cabin, but finds a blonde curl instead of a body. Donovan thinks it's all a publicity stunt, which is confirmed by the arrival of Mert Morgan, the studio press agent, and by Peter Dean, who turns out to be a beautiful woman, the same one who wore a blonde wig earlier. Finally realizing it was all a joke, Philip later tells "Pete" what tripe she writes, but when they return to his cabin they find the corpse of passenger Hendrick Van Mier. Trent summons Donovan again, who doesn't believe him, but when he finally examines the body he discovers that Van Mier's valuable White Dragon diamond has been stolen. As the ship is being searched, Philip wants to let Donovan handle the case while Pete wants to trace clues herself, especially those involving a Chinese woman, Li Tai, whose family had once owned the diamond. When Mert finds a bloody hand print, made by a man with a scar, they realize that it was made by "Cocky Joe" Ferris, whom Pete hired to play the corpse, and whom Donovan knows is a jewel thief. Donovan then leaves Pete and Philip handcuffed together and while they discuss the case, a masked man comes in with a gun demanding the diamond. Recognizing the set-up from one of her stories, Pete and Philip overtake the gunman, but when the lights go out he escapes and in the confusion, Pete accidentally knocks Philip unconscious. Helping Philip recuperate in her cabin, Pete prepares a drink for the two of them which turns out to have been drugged. Next morning, while everyone searches for the missing Donovan, Van Mier's valet, Williams, comes to Pete's cabin and brings Philip clothes to prevent gossip, after which Philip hires Williams. Before the boat docks in San Francisco, the diamond has been found and Trent is given credit, even though he doesn't want it. The diamond is switched for a hunk of coal, though, and the absent Donovan is revealed as a fake who has stolen the real diamond. Because the newspapers make Philip a laughing stock, Pete advises him to find the real murderer, whom she is certain is not Donovan. They find the body of Cocky Joe outside his room, but soon discover that he is only wounded and Joe tells Pete that Donovan is hiding in Chinatown, trying to sell the diamond. Chief Gibbs of the San Francisco police refuses to listen to Philip's new story, so Philip joins in on Pete's plan and talks to Donovan on the telephone, disguising his voice as Li Yat, the famous Chinese actor and husband of Li Tai. They arrange to meet Donovan at the Chinese theater at which Li Yat is playing. Meanwhile, the real Li Yat gets a note signed "Donovan" to meet him in the prop room. Donovan's dead body is later found by Philip and Pete in the prop room. It is finally revealed that Williams is the murderer and only posed as an English valet to get the diamond. With the help of Li Yat and his costumes, Philip is able to disarm Williams and, like the end of all his movies, ends up in the arms of the beautiful girl.