The Erymanthian Boar

Hercule Poirot is still in Switzerland after solving the third labour. Sightseeing, he takes a funicular to the mountain-top hotel of Rochers Neiges. On the way up, his ticket is checked by a conductor who passes him a hurriedly scribbled note that is from Lementeuil, the Swiss Commissaire of Police. It tells Poirot that he has been recognised because of his moustache and asks for his help. Marrascaud, a Parisian gangster, has fled from his homeland after the killing of Salley, a bookmaker, and is believed from information received to be having a rendezvous with members of his gang at Rochers Neiges. Poirot considers the note; Marrascaud has been the prime suspect in many killings but this is the first time that his guilt is beyond doubt. Although he is annoyed that his holiday is being delayed, the phrase used by Lementeuil to describe Marrascaud – 'a wild boar' – catches his interest. He sees in this the fourth of his self-imposed labours.

Poirot observes his fellow passengers in the funicular. There is a friendly American tourist called Mr Schwartz, a beautiful but melancholy woman, a distinguished-looking man reading a book in German, and three criminal types playing cards. Arriving at the hotel, they find it somewhat in chaos as it is only just opening at the start of the season. To Poirot, the hotel manager seems overly nervous. The only efficient person is Gustave, the waiter. Talking to the manager and then to Schwartz again, Poirot learns that the beautiful woman is a Madame Grandier, who comes each year on the anniversary of her husband's death in the area, and that the distinguished-looking man is Dr Lutz, a Jewish refugee from the Nazis in Vienna. Poirot introduces himself to Schwartz as Monsieur Poirier, a silk merchant from Lyon.

The next morning, when delivering coffee to his room, Gustave tells Poirot that in fact he is M. Drouet, a police inspector. They are now cut off in the hotel as the funicular has been damaged during the night, probably by sabotage. Poirot and Gustave discuss who Marrascaud could be among the guests and staff, which includes Jacques, the cook's husband. Poirot is intrigued as to why a rendezvous has been arranged in such an isolated place.

Poirot speaks to Jacques and his wife and discovers that before Gustave, there was another waiter, called Robert, who was dismissed for incompetence but who was not witnessed leaving the hotel. That night, the three card-playing men attack Poirot in his room, but he is saved by the pistol-carrying Schwartz. The three men are locked up and Schwartz tells Poirot that the men have already "carved up" Gustave's face. Schwartz and Poirot find Dr Lutz attending the not too seriously injured detective, and then follow a bloody trail down the carpets of the hotel to an unused wing where they find a dead body with a note pinned to it which reads "Marrascaud will kill no more, nor will he rob his friends." Poirot uses a heliograph to signal down the mountain for help and three days later, Lementeuil and some officers arrive at the hotel and Gustave is arrested.

Poirot announces that Gustave is not Drouet but Marrascaud. It was "Robert" who was Drouet; Marrascaud killed him and took his place. We learn that during his first night in the hotel, Poirot did not drink his coffee, as he suspected it was drugged, and actually witnessed Gustave entering his room, rifling his pockets, and finding the note from Lementeuil. The three card players were members of Marrascaud's gang but never attacked their leader; the carve-up of Marrascaud's face was carried out by Dr Lutz (who is a plastic surgeon and not a psychiatrist as he has pretended) to change his appearance. This is the real reason the rendezvous took place in such an isolated spot.