Pre-war Britain is striving to produce a new fighter aircraft. Minister for Armaments Lord Mayfield invites Air Marshall Carrington to his country home to discuss the secret plans. Due to the high security risk, Chief Inspector Japp has the estate patrolled by police but a vital design sheet is somehow spirited out of the house. Poirot, who is also a guest of Lord Mayfield, is on hand to track down the thief.