Arthur Simon Simpson (Peter Ustinov) is a petty thief who hangs out at the Greek airport swindling tourists. Here he runs into Walter Harper (Maximilian Schell), a sophisticated jewel thief who is planning to rob the Topkapi Museum in Istanbul of a bejeweled dagger. Harper and his companion, Elizabeth Lipp (Melina Mercouri), hire Simpson to drive their car, without them, from Athens to Greece. The Turkish border security finds guns in the car and, in order to free himself, Simpson agrees to spy for them. Eventually Harper hires him as his accomplice, even though Simpson is unaware of their plot to steal the dagger. He is not a brave man and, when he finds out his job involves being on the museum’s roof, he panics, but goes on with the plan with surprising results.