The story is set in Paris. The main character is Aristide Valentin, who is the city chief of police. Valentin has decided to stage an elaborate dinner party at his curious estate. The estate is notable because there is only one entrance, which is presided over by Valentin's fastidious servant, Ivan. Once in the house, visitors can arrive at a beautiful back garden. However, this garden is protected by a high wall; there is no entering or leaving, except by the main entrance.
Valentin invites many elite members of society, whom Chesterton describes in detail. Among the party-goers is an ambassador, a lady and her daughter, a priest, a doctor, and a soldier. These guests are described in quick succession, and, as a reader, it's a bit difficult to tell one from another. Also invited is Julius K. Brayne, a rich man from the United States, and Valentin's rival.
Chesterton includes some side plots here and there, including a tryst between the soldier and the young lady. These sorts of descriptions put characters in certain places when the real action begins to take place.
Everything changes when one member of the group finds a dead body in the garden. The head has been decapitated. Suspicion immediately falls on the solider, Commander O'Brien, who had been carrying a long sword. However, he claims that he had earlier put the sword down in another room and was not in possession of it at the time of the murder.
The guests begin to act as amateur detectives. The dead body is not initially identified as someone from the party. Eventually, they begin to put the pieces together. At one point, Ivan coincidentally brings inside the severed head of Julius Brayne, which was outside the house. As it turns out, the body in the garden is Brayne's, but the head is someone else's.
Eventually, we learn that, earlier in the day, Valentin had attended an execution by guillotine, and had brought the head back to his home. In the garden, he cut off Brayne's head after tricking the American into bending over. He then threw Brayne's head over the wall, and replaced the head next to Brayne's body in an attempt to fool the guests. The guests find out that Valentin is the murderer and attempt to approach him. Once they arrive at his study, however, they find him dead of self-poisoning