The international jewel thief known as the Ace of Diamonds has long been in retirement, but his protégé, Jeff Hill--dubbed the Jack of Diamonds--is rapidly building a reputation equal to his mentor's. After stealing a fortune in jewels from the New York apartment of Zsa Zsa Gabor, Jeff sets out to execute a robbery aboard a luxury liner. Though he encounters another burglar in one of the staterooms, he succeeds in escaping with the gems. Then, following the theft of Carroll Baker's jewels, Jeff flies to Munich and is visited by the Ace, who tries to persuade him to quit while he is still ahead of the game. But a chance meeting with Olga leads Jeff to an introduction to the master thief of Europe, Nicolai. He learns that Olga is the burglar he encountered on the liner and agrees to join forces with her and Nicolai in removing the fabled Zaharoff diamonds from an electronically-guarded bank vault in Paris. As the elaborate details of the crime are being worked out, Jeff cannot resist stealing the jewels of a third film star, Lilli Palmer. The Zaharoff job goes well, but, when an alarm is accidentally triggered and the police close in, the Ace appears and allows himself to be caught so that Jeff and Olga can escape. Jeff, however, refuses to accept the magnanimous gesture. A deal is negotiated with Von Schenk of the International Security Alliance whereby Jeff, Nicolai, and Olga agree to return all the jewels they have stolen--and retire from their criminal careers--in exchange for the Ace's freedom. The agreement is fulfilled, but Nicolai sheepishly admits that he has kept one small bauble for Olga's dowry.