When Mitchell, an American freeboot "security man", gets the upper hand he learns his assailant is "Smith", an investigator for the insurance company that had written a $1 million dollar policy on a "priceless" artwork, Leonardo da Vinci's Madonna and Child, that had been part of the ship's cargo, on loan from the U.S. to appear in an upcoming exhibition at London's famed Tate Gallery.

Mitchell explains that he has been hired by the painting's owner to ensure its safe delivery, and is courteously introduced to Sir Richard Aldingham by Tate director Peter Randall, described as one of Britain's great art collectors and museum benefactor who had been instrumental in arranging the exhibition.

Mitchell, ever cheeky and self-advancing, cheerfully unwraps the parcel he'd had tucked under his arm and declares it the original artwork, which he'd hidden in the ship's safe anticipating a theft attempt.

Rhapsodizing over a potential $50,000 reward that has been posted, he then announces his intention to stay on and attempt to discover how the foiled effort was linked to recent successful heists of Leonardos in Florence and New York – in each instance the original having been replaced by a forgery so nearly perfect it had delayed discovery of the switch for weeks.

In spite of her prickly rebuffs, Mitchell persists in pursuing Mary, making few obvious gains in his work but beginning to win Mason over, whom he suspects is the closet genius who painted the Leonardo forgeries.

Infuriated that his catspaw disobeyed orders and both phoned him there then showed in person, Sir Richard then demands that Weston murder Mary to prevent her from raising suspicions.

Although the ending is plainly evident from the first meeting with an oh-so-terribly obliging Sir Richard Aldingham – too silkily played by Hugh Williams – the script avoids the more obvious thriller tricks and achieves a decent suspense throughout.