Melina Mercouri (A man could get killed) 1966

An American businessman who is mistaken by everybody for a super-secret British agent on the trail of smuggled diamonds. When supposedly his liaison at the British Embassy meets him at the Lisbon airport, the car intended for him is blown up. No matter how much he protests, all of which are accepted as cover for his actual mission, he is then transported into a world of scheming, dangerous, underworld characters.