The story begins when Hercule Poirot visits Dr. John Stillingfleet, a well-known dentist in London. Poirot has an appointment for a routine check-up, but during the visit, Dr. Stillingfleet is shot dead right in front of him. The murder appears to be an obvious suicide at first, as Dr. Stillingfleet is found in his office with a gunshot wound, and the door is locked from the inside.