I Try to Solve a Japanese Honkaku Murder Mystery: The Decagon House Murders, by Yukito Ayatsuji

DecagonHouseMurdersSo I decided to take a mini-break from Agatha Christie and try my hand at solving a case from a different writer. After all, I didn’t want to get so used to Christie’s style that I could pick out her storytelling tricks and solve her mysteries with my eyes closed. (Hah!)

Seriously, though, I’d heard of a Japanese literary movement called honkaku (and later, shin honkaku) mysteries. Writers within this genre take their inspiration from Golden Age detective fiction. Like Christie and her peers, these Japanese writers followed the principle of ‘fair play’ — they crafted intricately woven puzzles, but provided all the clues within the text, so that the reader had everything they needed to solve the mystery themselves. Crime Reads has a great primer to the genre.

For my first honkaku mystery, I selected The Decagon House Murders by Yukito Ayatsuji. It’s a locked room mystery that clearly takes its inspiration from Agatha Christie’s And Then There Were None. Seven university students, who all belong to a club of mystery-lovers, travel to a remote island to spend a week at the Decagon House. The house, so-called for its shape, was built 20 years ago by a reclusive architect as an annex to his own home, the Blue Mansion. Six months before the students go to the island, the architect, his wife, and two servants are all murdered, and the Blue Mansion is burned down. The island was then sold to someone whose nephew is in the mystery club, and the students decide it would be a great spot for their get-together.

Except that the second day of their trip, they wake up to find seven plastic nameplates with the labels First Victim, Second Victim, and so on to Fifth Victim, with the final two plates labelled Detective and Murderer. And then the deaths proceed from there.

As a piece of literature, it’s top-notch. The book is a love letter to Golden Age Detective Fiction. The character names are based on famous writers; the students have taken on mystery writer nicknames, like Ellery, Agatha, and Poe. Characters also geek out over mysteries; our first introduction to the students has Ellery talking about how strongly he prefers mysteries that are intellectual puzzles, and Carr accuses him of being elitist. Even the killer in the first chapter reflects on how the best plan is one that allows for flexibility, and considering that they also reference And Then There Were None, I think that comment is a tongue-in-cheek critique of how rigid Christie’s killer was, having to adhere to the details of a children’s nursery rhyme.


New case, new writer, new detective notebook.

As a mystery (and I write this 82% in), it’s pretty good. The story follows two separate yet intersecting cases: the quadruple murder of the architect and his household six months ago, and the murders of the students in the present-day. The students on the island are desperate to identify the killer, for obvious reasons, yet the novel lacks the claustrophobic quality of And Then There Were None. I think part of it is that the scenes on the island are interspersed with scenes on the mainland, where two former club members receive mysterious letters that has them looking into the quadruple murder. So even as the students on the island have no idea why anyone may be after them, we readers are well-aware, because of the mainland story.

Still, the mystery is solid. Some of the murders appear completely random: a poisoned cigarette could have been used by anyone, and same for a poisoned coffee cup when all the cups are identical. So it’s tough to see which of the students, if any, put themselves at such risk. So as a reading experience, it did feel very much like, as Ellery calls his favourite mysteries, an ‘intellectual puzzle.’

Now, as a mystery I’m actively trying to solve, it can feel very frustrating — and I mean that as a compliment. The author had me second- and third-guessing myself throughout. No sooner would i come up with a theory than one of the characters would voice it themselves, and then I’m left to wonder: does that mean the theory is a red herring, or is it actually still valid, and the author is trying to psyche me out by bringing it into the open? One of the characters was firm on a rather far-fetched theory that, to be fair, would’ve been a very valid option as a surprise twist given what we’re expecting based on our experience with Christie’s book. But then that character brought it up so often and so adamantly that again, I’m thinking, obviously, that can’t be the answer. …Or can it? A big challenge with trying to solve a mystery that is so clearly an homage to Golden Age Detective Fiction is that you know that it’s not just the writer who knows all the tricks, but the characters do as well. So they could very well be subverting tropes just for funsies.

Even the mainland investigation into the quadruple murder has confused me. On the surface, that mystery has been neatly solved and wrapped in a bow. The problem is, I don’t know if I should trust these characters. Unlike, for example, Agatha Christie’s Cards on the Table, where her detectives are series characters so you know you can take them at their word, the detectives here are all unknown to me. There’s an eager beaver character who inserts himself into the former club members’ investigation, and in some ways ends up running it. And that kind of character is either totally suspicious, or being set up to be an eccentric series character. I just don’t know.

All to say, I do have a guess. Do I feel confident in it? Not at all. Do I have other theories I also feel strongly about? Sure, but my gut isn’t necessarily pointing me in any direction that makes sense. To be honest, I don’t really feel too strongly about any of the characters; to me at least, this story very much remains an intellectual puzzle rather than a gripping page turner. Mostly I feel confused. But the deaths have occurred, and the mainland characters have been called together, so I feel like the big reveal is coming up. So here we go!


Okay, this got me. From my rather detached ‘intellectual puzzling’ through it all, to a legit gasp at the big reveal. Wow.

Did I guess it? No, not at all. Or rather, my gut actually did lean towards the right answer, but then my mind got in the way and reminded me that my gut feels had no rational backing, so I went with the answer that made sense but turned out to be wrong. Bah.

So, okay, well done, Yukito Ayatsuji. Bravo.


My Theory / My Detective Big Reveal:

First, the Characters:

On the mainland:

  • Former mystery club members: Kawaminami Takaichi (Conan Doyle), Morisu Kyoichi
  • Eager beaver friend of architect’s brother: Shimada Kiyoshi
  • Architect’s brother: Nakamura Kojiro

Mystery club members on the island: Ellery, Carr, Leroux, Agatha, Orczy, Van Dine, and Poe

Next, the Clues:

  • The architect had a daughter, Nakamura Chiori, who also belonged to the mystery club. In January of the year the architect got murdered, Chiori died during a mystery club afterparty (alcohol poisoning or an overdose, I can’t remember which exactly). Kawaminami and Morisu were not at the afterparty.
  • All nine mystery club members received letters saying “My daughter Chiori was murdered by all of you.” (Unsure if the seven on the island ever saw the letter.) Kojiro received a letter saying “Chiori was murdered.”
  • Van was the first on the island. His uncle owns the place, and when the other students arrived, he had a cold. Which seems suspicious for some reason. Like, was he swimming around setting up his traps? Still, none of the murders actually requires an elaborate setup; any of the seven could’ve done them.
  • Orczy was actually a close friend of Chiori, and the only one who seems to remember her. When she is killed, only Poe (her best friend since childhood) sees her body, and he actively keeps the others from coming closer. Could she still be alive?
  • Ellery is very much into intellectual puzzles. He keeps doing magic card tricks, sleights of hand where it appears he’s reading your mind.
  • Kojiro said Chiori was actually his daughter, not Seiji’s. Seiji’s wife Kazue and Kojiro were secretly in love. Shimada says Kojiro and Seiji didn’t actually get along.
  • Kazue’s left hand was removed after she was killed. In the present day, Orczy’s and Carr’s left hands were also removed, but only Carr’s was seen by multiple people.
  • When Seiji the architect and his household died, the fifth member of the household, a gardener, disappeared. Seiji’s body was burned; was it actually the gardener’s body? And did Seiji actually escape? –> However, in the present day, Ellery and Van come across a decomposing body beneath Decagon House. So if that’s Seiji/the gardener, why would they murder the other four and then just end up in the cellar themselves to die?

Okay, my Conclusion:

I think Orczy and Ellery are in cahoots on the island. Orczy clearly wants revenge for Chiori’s death. I have no idea about Ellery’s motive, only that the risky murder methods and trickiness of everything feels very much like him. He also seems the de facto leader of the group, and I think he’s been pulling strings the entire time.

Like, I feel like Van Dine had the most opportunity coz he was there first, and his being sick feels sus, but he also has no motive that I can see? Neither does Ellery, but the trickery of it all just feels like him?

I think Orczy is still alive, and secretly helping Ellery out behind the scenes. I think she got Poe to cover for her by telling him she could go undercover as a detective if everyone thought she was dead.

I think the body beneath the Decagon House is Seiji’s. I think he killed his wife and household, and then maybe called his brother over to gloat. Then I think Kojiro killed Seiji and then put his body in the Decagon House secret room so everyone would still think Seiji or the gardener was the murderer.

I feel like someone on the mainland is involved as well. My gut says maybe Morisu because he’s so quiet and serious, but I can’t figure out a motive, so I’m going with Kojiro, to avenge Chiori’s death. He sent the letters to everyone, and then kept the original for himself, only cutting out the phrase “Chiori was murdered” to remind himself of why he’s doing all of this.

The Actual Reveal:

Van Dine was the killer. His real name is actually Morisu, he was secretly in a romantic relationship with Chiori, and he basically secretly travelled between the island and the mainland to feature in both halves of the story.

The body under the Decagon House is the gardener, whom Seiji likely killed and hid there, or the gardener tried to escape but Seiji killed him there.

1 thought on “I Try to Solve a Japanese Honkaku Murder Mystery: The Decagon House Murders, by Yukito Ayatsuji

  1. Pingback: I Try To Solve a Japanese Honkaku Murder Mystery: The Honjin Murders, by Seishi Yokomizo | Literary Treats

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