The Decagon House Murders

The novel, a work in the Japanese honkaku, or fair-play puzzle mystery, tradition, alternates between two storylines: one set on a remote island where members of a university mystery-fiction club are murdered one by one, and another on the nearby mainland where former club members investigate a threatening letter. The dual structure conceals a central twist that reframes everything the reader has been told.

In a brief prologue, an unnamed man sits on a breakwater at night, contemplating revenge. He has designed a flexible framework rather than a rigid scheme, describing his trap as having ten equal sides and interior angles. He intends to kill his victims sequentially, maximizing their terror. Before leaving, he throws a sealed green glass bottle containing an unsigned confession into the sea.

Six members of K— University's Mystery Club travel by fishing boat to Tsunojima, a small, cliff-bound island off the coast of Ōita Prefecture in southern Japan, to join a seventh member, Van, who arrived earlier to prepare supplies. The members go by nicknames from famous mystery writers: Ellery, Carr, Leroux, Poe, Agatha, and Orczy. They stay in the Decagon House, an annex whose outer walls form a ten-sided polygon with ten rooms surrounding a central hall. The island's main structure, the Blue Mansion, burned down six months earlier during a quadruple murder: The architect Nakamura Seiji, his wife Kazue, and two servants were found dead. Kazue had been strangled and her left hand severed. A gardener named Yoshikawa Sei'ichi vanished and became the prime suspect. Van, who has a fever, retires early each evening.

On the mainland, Kawaminami Taka'aki, a former club member, receives a typed letter signed with Seiji's name accusing the club of murdering Seiji's daughter. Kawaminami recalls that Nakamura Chiori, a first-year student, died of acute alcohol poisoning at the club's after-party the previous January after being pressured to drink. Kawaminami and his close friend Morisu Kyōichi had left the party before the death occurred. Kawaminami discovers that all the after-party attendees received identical letters and have departed for Tsunojima.

Kawaminami visits Nakamura Kōjirō, Seiji's younger brother, who dismisses the letters as pranks. At Kōjirō's home, Kawaminami meets Shimada Kiyoshi, Kōjirō's university friend, who takes an interest in the case. That night, Kawaminami and Shimada visit Morisu at his apartment. Morisu, an introverted young man, proposes that the case may involve a "Birlstone gambit," a mystery-fiction trick in which one of the supposed victims is actually the killer. He argues that Seiji's body, too badly burned for reliable identification, could have been switched with the gardener's. Shimada theorizes that Seiji's motive was jealousy over an affair between Kazue and Kōjirō. Morisu withdraws from active investigation but agrees to serve as an "armchair detective."

Back on the island, the second morning brings a sinister discovery: seven white plastic plates on the central table, labeled for five victims, a detective, and a murderer. All seven members deny responsibility. Ellery privately tells Leroux he believes the plates are genuine murder announcements.

The killings begin on the third day. Orczy is found strangled in her bed, with the first victim's plate affixed to her door. Her left hand has been severed, echoing the Blue Mansion case. That evening, Carr collapses after drinking coffee and dies of poisoning.

On the mainland, Kawaminami and Shimada visit the missing gardener's wife, who reveals Seiji's fortune was nearly depleted, undermining the robbery motive. Shimada later confronts Kōjirō, who confesses that he and Kazue had a night together and Chiori was the result. Kōjirō also reveals that on September 19th, he received Kazue's severed hand in a package and called the mansion; Seiji answered calmly, speaking of death as a blessing. Kōjirō insists the psychology proves Seiji planned a murder-suicide. Kawaminami and Morisu accept this conclusion.

On the island, the murders accelerate. On the fourth day, Ellery discovers a concealed underground room beneath the Blue Mansion ruins but trips over a fishing-line trap and sprains his ankle. On the fifth morning, Leroux is found beaten to death in the ruins, Agatha is poisoned by prussic acid on her lipstick, and Poe dies after smoking a cigarette laced with potassium cyanide. Only Ellery and Van remain.

Ellery analyzes footprints at the scene of Leroux's murder and concludes the killer came from the sea, not from the Decagon House. He declares the murderer must be Seiji, seeking revenge for Chiori's death. He then notices that the cup used to poison Carr has 11 sides rather than 10, concealed by the building's decagonal design. Reasoning that an 11-sided object implies a hidden room, he uses the cup as a key to open a secret staircase beneath the kitchen. In the underground passage, they find a half-decomposed corpse Ellery identifies as the gardener. He plans to capture Seiji when the killer returns.

Van prepares coffee for both of them. Ellery drinks it and falls asleep, drugged by sleeping tablets Van dissolved in the cup. Van douses the building in kerosene and sets it ablaze.

On the mainland, Morisu learns everyone on the island is dead. He contacts Kawaminami and Shimada, and all three travel to S— Town. Inspector Shimada Osamu, Shimada Kiyoshi's older brother, leads the investigation. The police theorize that Matsu'ura Junya, the student known as Ellery, murdered his five companions before setting fire to the building and himself.

Days later, at a meeting in the club room, Inspector Shimada asks about the club's nickname tradition. Kawaminami identifies himself as Doyle. The inspector guesses Morisu's nickname is Maurice Leblanc. Morisu quietly corrects him: He is Van Dine.

The narrative reveals the full truth. Morisu is the seventh person on the island, the one called Van. He has been Chiori's secret boyfriend. Having lost his parents and sister to a robbery in middle school, Chiori's death drives him to plan revenge against the six students who pressured her to drink. He engineers the trip after learning his uncle has purchased Tsunojima, ensuring outsiders believe only six students are going while he joins the group as Van. Each night, he travels by rubber dinghy to the mainland to meet Kawaminami and Shimada, building his alibi. He writes the threatening letters under Seiji's name to create the impression that the dead architect is alive. He kills Orczy first because the ring on her finger bears his and Chiori's engraved initials; unable to remove it, he severs the hand and frames the mutilation as an allusion to the Blue Mansion case. He poisons Carr's cup with arsenous acid, smears prussic acid on Agatha's lipstick, beats Leroux to death when Leroux discovers his boat, injects cyanide into one of Poe's cigarettes, and drugs Ellery before setting the building ablaze.

In the epilogue, Morisu sits alone on the breakwater, calling out to Chiori's memory, but her image will not appear to him. Shimada approaches and implies he has deduced the truth. Morisu deflects and walks away, then discovers at the water's edge the green glass bottle from the prologue: his own written confession, washed back to shore. With a faint, bitter smile, he gives the bottle to a passing boy and asks the child to deliver it to Shimada. The novel ends with Morisu surrendering himself to judgement.