In the spring of 1939 in England, Oxford University Professor Richard Myles (Fred MacMurray) and his new bride Frances (Joan Crawford) decide to honeymoon on Europe. Because they are American tourists and therefore "above suspicion," they are commissioned by the British secret service to find a scientist who has developed a countermeasure against a new Nazi secret weapon, a naval mine. Without knowing his name, what he looks like, or where to find him, the couple look upon the search as an adventure and cross Europe seeking clues from clandestine contacts.