Elizabeth Lipp (Melina Mercouri) visits Istanbul, where she sees a traveling fair featuring replicas of treasures from the Topkapı Palace.

Leaving Turkey, she recruits her ex-lover, Swiss master-criminal Walter Harper (Maximilian Schell), to plan a theft of the dagger.

They engage Cedric Page (Robert Morley), master of all things mechanical; Giulio, "The Human Fly" (Gilles Ségal), a mute acrobat; and the burly Hans (Jess Hahn), who will provide the muscle needed for the job.

Harper and Lipp then hire small-time hustler Arthur Simon Simpson (Peter Ustinov) to drive a car into Turkey to transport hidden explosives and firearms for use in the burglary.

Hans' hands are injured in a scuffle with the drunken cook, Gerven (Akim Tamiroff), and Simpson is engaged as a substitute, prompting him to confess that the police are watching them.

Knowing they face arrest if they try to escape Turkey, or use their equipment, Harper improvises a new plan in which they will give the still-oblivious police the slip, and steal the dagger without using their weapons.

When word of the Topkapı alarm reaches the police, Major Tufan confronts the thieves, displaying Simpson's last note, which has just enough information to link all of them to the theft.

Ultimately, the gang is seen in a Turkish prison, where Lipp begins to tell them of her fascination with the Russian Imperial Crown Jewels in the Kremlin.

Appearing in supporting roles were Gilles Ségal as the “human fly" and Joe Dassin as Joseph, who runs the traveling fair display that is supposed to smuggle the dagger out of Turkey.

[5] Bosley Crowther of The New York Times applauded the film as well acted, "adroitly plotted," and abounding in brilliant Technicolor that Dassin exploits "like a child with a new paint box," but he saved his greatest praise for Ustinov:[I]t is his misadventures and confusions and frights that truly make this picture something more than melodrama with a farcical edge.