Plot
A cargo ship unloads a clutch of nondescript wooden cases onto the nighttime London docks. A fracas is instigated, and in the ensuing chaos a shady figure makes off with one of its crates.
A man watching it all from a distance, Paul A. Mitchell (Dennis O'Keefe), takes off after him, but is tackled by a seeming conspirator. They struggle. When Mitchell, an American freeboot "security man", gets the upper hand he learns his assailant is "Smith" (Guy Middleton), 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. As the thief and his helpers have separately disappeared, the pair head straight to the Tate and awaiting museum authorities there.
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 (Hugh Williams) by Tate director Peter Randall (Gerald Case), described as one of Britain's great art collectors and museum benefactor who had been instrumental in arranging the exhibition. Randall's politeness ends there, excoriating Mitchell for allowing the DaVinci to be stolen. 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. He then asserts that there had to have been a mole at the Tate, as the cargo manifest did not identify the painting yet the thieves knew exactly which crate to steal.
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was linked to recent successful heists of DaVincis in Florence and New York - in each instance the original having been replaced by a forgery so nearly perfect it had delayed discovery of the switch for weeks.
Mitchell's first suspect becomes minor but highly attractive Tate employee Mary Mason (Coleen Gray), who sneaks a shady looking man into the exhibition opening through the gallery's basement. This falls flat with everyone when it's revealed it was just her father, Henry Mason (John Laurie), a talented but opinionated minor painter known to museum authorities. 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 DaVinci forgeries.
Thanks to the Tate's extremely inadequate security measures the Madonna and Child is soon stolen, with the thief eluding a less than aggressive pursuit by Mitchell. Following an evening of courting Mary, Mitchell is roughed up by two thugs, but saved from worse by a providentially trailing Smith. His new ally invites him to inspect Henry's quayside lair, where the loner paints in anonymity...or secrecy. Which it is they hope to determine, but instead discover him strung from a noose in an apparent suicide.
Mitchell knows enough to persuade Smith it was actually murder, and in a second visit there accidentally stumbles onto a clue that may help him home in on the kingpin behind it all.
Meanwhile, the adventurer carrying out the DaVinci thefts, Weston (Seymour Green), appears at the home of Sir Richard, catching the connoisseur entranced in his three stolen masterpieces. 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. When Weston refuses and threatens betrayal in return, Sir Richard deftly pours poison into his drink and sends him on his way...to a gasping death in a callbox shortly after.
He then convinces Mary to come to his home with a promise of taking her to grieve in companionable comfort at his sister's. Before he can kill her, Mitchell appears from the shadows and threatens to destroy the Madonna and Child with a vial of acid. Holding the lunatic at gunpoint, Sir Richard berates him for proposing to destroy an irreplaceable masterwork. Tossing on the acid distracts Sir Richard, who drops his gun and rushes to the painting. Mitchell calmly takes the pistol retrieved by Mary, explains to Sir Richard he'd been outfoxed, that Mason had actually stolen a forgery, and holds him for arrest.
With the original safely on display at the Tate the next day, a beaming Mary is tumbling toward matrimony with the soon to be flush maverick, less her father but with all the other loose ends tidied.
