Elizabeth Lipp (Melina Mercouri) visits Istanbul, where she sees a traveling fair featuring replicas of treasures from the Topkapı Palace. Next she cases the Topkapi, fascinated by the emerald-encrusted dagger of Sultan Mahmud I. 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.