Melina Mercouri (sometimes I see her looks are scary) plays Elizabeth Lipp, a beautiful thief without a single police's record. She wants a dagger once belonged to Sultan Mahmud I of the Ottoman empire, which holds 4 priceless emeralds. This dagger is stored in Museum Topkapi in Istanbul. She contacts Walter Harper (Maximilian Schell), a thief without a police record, also her former lover, to do the job for her. Walter agrees, but he wants to recruit amateurs as his crew because after the theft is done, the police will search among notorious thieves.
Walter recruits 1) Cedric Page (Robert Morley), a toy maker, to deal with the alarms 2)Giulio (Gilles Ségal), a human fly, to trade the real dagger with a fake by hanging from a rope 3)Hans Fisher (Jess Hahn), a strong man, who will hold the rope; and 4) Arthur Simpson (Peter Ustinov), a tourist cheater, who brings the car which hid a rifle to shoot the lighthouse's lamp (so the museum guards cannot see Giulio) and smoke bombs to divert the guards. Unfortunately, when bringing the car from Greece to Turkey, the rifle and smoke bombs are found by the customs and the authorities mistake the group of thieves as terrorists. They let Arthur Simpson go, but he must act as their spy.
Like any good heist movie, the heist itself must be a success. So Walter Harper and his friends splendidly succeed and the audience are satisfied to watch how the theft is done. It was a breathtaking moment, really, especially if you see it for the first time, like me. Then, as predicted, the gang are caught because there is an idiot among them: Arthur Simpson.