Detective Lieutenant Tony Ricco of the homicide squad becomes frustrated when an unidentified corpse is found on a beach, and his investigation yields no clues about the dead man's identity. As he is contemplating the case, he is joined in his office by Inspector Geoffrey James of Scotland Yard, an old friend who is visiting from England. Tony asks for Geoff's help, as the dead man, who was wearing a Harris Tweed coat and carrying pound notes, is apparently English. Geoff then goes to the morgue, where he discovers that the corpse's coat is covered with fluorescein dye. Geoff explains to Tony that the dye, which was developed during the war, is used as a sea rescue aid, because it glows brightly and helps planes spot people floating in the water. The dead man did not have any water in his lungs, however, so Geoff surmises that he must have picked up something from the water, such as a life preserver, and put it on over his coat. Geoff then examines the area where the man was found and discovers a beachcomber's shack. There, Geoff finds a hidden life preserver, and after putting it on under his coat, is interrupted by a mysterious woman and the belligerent beachcomber. The man attacks them, but Geoff easily extricates himself and the woman from the situation. She refuses to tell Geoff who she is and attempts to escape from him. While pursuing her, Geoff is knocked unconscious by her confederate, and the life preserver is stolen. Theorizing that the preserver was tossed from a passing airplane and then retrieved by a seaplane, Geoff investigates trans-atlantic flights until he finds one on which a bathroom window was broken en route. Anne Hogan, the air hostess on the flight, accompanies Geoff to the morgue and identifies the dead man as Mr. Rodine, a British diplomatic courier who passed through English customs without having his bags checked. Tony then reveals that he is aware that Geoff's true purpose in visiting America is to trace the gang behind a rash of jewel thefts in London. Geoff admits he suspects Rodine of having brought the jewels aboard the plane and then tossing them out the window after attaching them to the life preserver. Geoff contacts the British consul, who informs him that the real Rodine was killed in London before the airplane flight occurred. With the corpse still unidentified, Geoff and Anne go in search of the seaplane used to retrieve the jewels and learn that pilot Joe King flew the man to the jewels. During their investigation, Geoff runs into the woman from the beachcomber's shack, and Anne identifies her as Irene Trilling, a fellow passenger of Rodine's impostor. Irene escapes again, but Anne recognizes her companion as Frank Farrington, who was also on the flight. The dead man is finally identified as con man Louis Gannet, and Joe King, who has been beaten by Farrington, confesses to helping Gannet get the life preserver. King states that he saw Farrington kill Gannet, and Farrington then accused him of stealing the missing jewels from Gannet. Geoff deduces that the beachcomber must have picked up the jewels while Farrington and Gannet were fighting, and while searching the shack, Geoff finds the loot hidden in a lobster tank. Geoff then goes in search of Anne, who has found out that Farrington and Irene are hiding in a nearby deserted house. Farrington, who has realized that Irene and Gannet were attempting to double-cross him, shoots and kills Irene, and attempts to kill Geoff when he arrives. Geoff is saved by police sergeant Blake, who has been posing as a drunk while surreptitiously acting as Geoff's bodyguard. Later, Anne rewards Blake and Geoff for their efforts with kisses, which Blake calls "intoxicating."