Tommy Mayfield is a British industrialist who has developed a new fighter plane. Up to now he has paid all development costs himself and he very much wants the government to contribute. It is reluctant to do so because of an apparent indiscretion some years before when Mayfield was supposed to have sold heavy artillery to the Japanese. In order to get back into the government's good books, he decides to lay a trap for Mrs. Vanderlyn, a known Nazi sympathizer and possible spy. The bait is the plans for the new fighter but Mayfield's wife is so concerned that she asks Hercule Poirot to spend the weekend at their house to make sure Vanderlyn doesn't get away with anything.