The Mill House Murders is the second book in Yukito Ayatsuji's Mansion Murders series. Amateur detective Kiyoshi Shimada visits another strange mansion designed by eccentric architect Seiji Nakamura, this one belonging to Kiichi Fujinuma, the reclusive son of a famed artist, who keeps both his father's paintings and his young, beautiful wife locked up away from the world.
One year prior to the book's events, a young monk named Tsunehito Furukawa, whom Shimada considered a friend, is suspected to have killed two people and vanished with one of Issei Fujinuma's valuable paintings. Not believing that his friend is guilty, Shimada seeks the truth of what really transpired in the Mill House one year ago, but as he begins unraveling the mystery, a new series of murders begins...
This work contains examples of:
- Age-Gap Romance:
- Kiichi Fujinuma apparently fell in love with Yurie during the decade or so that he spent as her guardian and he married her once she was of age. He seems to genuinely care for her, though other characters comment on their relationship with varying levels of disgust. Considering Yurie became Shingo Masaki's lover and accomplice, she doesn't seem to have been as fond of her guardian-turned-husband.
- Speaking of Shingo Masaki, he's the same age as Kiichi, in his late 30s, when he gets involved with the 18-year-old Yurie. She apparently does return his feelings, at least for a time.
- Asshole Victim: Mitamura is shown to be a massive creep towards Yurie, all but outright forcing himself onto her out of lust. It's thus not exactly sad when Masaki kills him, despite him being innocent of any outright crimes.
- Career-Ending Injury: Masaki's career as an artist was destroyed after the car accident he was caught in left him colorblind.
- Dead All Along: Both Furukawa and Kiichi turn out to have died in 1985 as part of Masaki's plan to steal the latter's identity.
- Even Evil Has Loved Ones: Despite being overall a fairly odious killer, Masaki does genuinely love Yurie.
- Faking the Dead: Shingo Masaki killed Tsunehio Furukawa, chopped up his body, and burned it in the Mill House's furnace, after cutting off his own finger which bore a distinctive ring, in order to frame Furukawa for his own crimes and stage the "disappearance" of the priest.
- Foreshadowing:
- At the beginning of the novel, housekeeper Tomoko informs her employer that there is a foul smell coming from somewhere in the basement, but he brushes her off. The real Kiichi Fujinuma's body has been hidden in a secret room down there for the past year.
- In the past chapters, Masaki tells both Furukawa and Mitamura that he has been unable to paint since the car accident that killed his girlfriend and rendered Kiichi paraplegic, suggesting that his artistic spirit was killed by grief. He also hints at having held onto resentment towards Kiichi for the last twelve years due to that. He actually went red-green colorblind due to brain damage caused by the car accident, and all of this, plus his money troubles and his love for Yurie, form his motive to kill and impersonate Kiichi.
- Multiple mentions are made in the 1985 portion concerning the extensive medication Kiichi is required to take for his health, of which no mention is made in the 1986 part. Since the "Kiichi" in the latter is actually Masaki, no such medicine is required.
- When Shimada presents Kiichi with the threatening note he found on the floor, he mentions that the green note on the red carpet looked like a stain to him at first; Kiichi muses on the strangeness of that, mentioning that there is no way he should have have missed it thanks to being wheelchair-bound making his eye level lower. This also hints at Masaki's colorblindness and his impersonation of Kiichi.
- While observing the quirks and tics of the various guests, Shimada notes that Kiichi always holds out his little and ring fingers perfectly even when holding his pipe. The difficulty of doing this so perfectly every time tips him off to the fact that "Kiichi" is actually missing the finger that Masaki lost.
- Much attention is made about the beautiful view of the sunset and surrounding forest from the Mill House, noted as being one of its best features. An eagle-eyed reader might notice that not once does "Kiichi" ever describe this in his narration, a big hint to Masaki being colorblind.
- Guilt-Ridden Accomplice: Yurie assisted Masaki in his murders by helping give him alibis for his crimes, but only took part in them because she was too naive to fully grasp the morality of what she was doing. Her newfound maturity in the climax is what causes Masaki to undergo his Heel Realization.
- He Knows Too Much: Why Fumie the housekeeper had to die: Since she was an ex-nurse who insisted upon treating Kiichi personally, she likely would have seen through the culprit's plan to Kill and Replace him if she'd been kept alive.
- Hypocrite:
- Early on, Mitamura tells Mori that he is disgusted by Kiichi marrying Yurie due to their age gap and the fact that he was once her guardian. That doesn't stop him from making a pass at Yurie himself and also inviting himself up to her room, despite being decades older than her just like Kiichi.
- The culprit, Masaki, recruits Yurie as an accomplice by calling Kiichi selfish and a creep for keeping her locked up in the manor all her life, only to do the exact same thing after the murders.
- It's Personal: Shimada was a friend of Furukawa in university, and thus knew him to be someone who would never stoop to murder. This, combined with the immensely suspicious circumstances behind his disappearance, is what motivates him to investigate the case and clear his friend's name.
- Kill and Replace: The real Kiichi Fujinuma was murdered a year ago. The Kiichi who's been rolling around since is Shingo Masaki, who killed Kiichi and stole his identity due to coveting his wealth and wife. Shimada Lampshades how easy Kiichi made it, with his mask, loose fitting clothes, raspy voice, and using a wheelchair that hides his height.
- Locked Room Mystery: An entire locked floor: Tsunehito is on the second floor, then he isn't. The only stairs down from where he was were observed by two people, and none of the windows up there open more than a few inches. Masaki killed him, then took him in the bathtub and chopped him in to pieces small enough to fit through the gap in the window. He wanted to frame Tsunehito for the crimes by making it look like he'd run away; the witnesses turning his disappearance from "suspicious" to "impossible" weren't part of the plan.
- Mad Artist:
- Seiji Nakamura, who always designed houses with strange gimmicks and secrets, and eventually murdered his wife and servants, is once again mentioned throughout the book, given that the mystery takes place in another house that he designed.
- Issei Fujinuma reportedly painted subjects that came to him in visions, and his last painting, The Phantom Cluster apparently frightened both himself and his son so much that it has never been displayed publicly. At the very end, the painting is finally viewed, and it seems that at least some of Issei's paintings depicted visions of the future; The Phantom Cluster is full of symbolism that mirrors the events of the book itself, which take place long after Issei's death.
- Maybe Magic, Maybe Mundane: The Phantom Cluster, Issei Fujinuma's final work, is full of imagery connected to the events of the book, despite having been painted well before any of them took place. Most of it can be explained by his son Kiichi deliberately following the path laid out for him, but the painting also predicts events caused by the culprit, who has never seen the painting before...
- Murder the Hypotenuse: Done twice by Shingo Masaki, though the first attempt is ambiguous until the very end of the book. He murders Noriyuki Mitamura, the hospital director, after the latter invites himself up to Yurie's room with the obvious intent of a dalliance with her. The desire to have Yurie to himself also played a part in his motive to kill Kiichi Fujinuma and assume his identity.
- No Ending: The book ends very abruptly with Masaki discovering The Phantom Cluster to in-fact be a painting prophesizing the entire tragedy of the story, with no resolution to what happens after.
- Obfuscating Disability: The able-bodied Shingo Masaki has been impersonating paraplegic Kiichi Fujinuma for a year, staying confined to a wheelchair in front of everyone except Yurie, his accomplice. In a slight aversion however, Masaki is hiding his actual disability at the same time; namely that the accident 13 years ago left him red-green colorblind.
- That Man Is Dead: From Masaki's perspective, the man named "Masaki Shingo" died one year ago, replaced by the life of Kiichi Fujinuma which he stole. This is used to help mask The Reveal through Exact Words.
- Too Much Alike: The culprit deeply hates Kiichi Fujinuma despite being extremely similar to them. They both had their lives ruined in the same car accident, which gave them disabilities, and they both covet the much younger Yurie. The first chapter of the present day section and the first chapter of the past section even describe their morning routines in nearly identical terms, complete with both of them taking time to look at their reflections and curse their faces.
- Unreliable Narrator: Kiichi Fujinuma is the first person narrator of the present day half of the book, and he's got plenty of issues that color his perception. While he never intentionally lies to the reader, he does pick his words to avoid bringing up his own character flaws, that he's literally having his perception colored by his colorblindness, the murders he commits, or that he's actually Shingo Masaki after having killed the real Kiichi and stolen his mask and clothes.
- Villain Protagonist: "Kiichi"/Masaki is the culprit of the story, killing five people in his plan to steal Kiichi's life and subsequently hold onto it.
- Wham Line: Said to the culprit by Shimada right before both go on to explain just how and why the murders occurred: "Are you going to make me take that mask off your face myself, Masaki Shingo?"
- White Mask of Doom: Kiichi Fujinuma's face was disfigured in the car accident that also paralyzed him. Since then, he has always worn a white rubber mask which covers his entire head, leading his home to also be nicknamed "Mask Manor" after its owner's unsettling appearance. The person actually wearing the mask for most of the book is Shingo Masaki, the murderer behind all of the events.
- Wife Husbandry: Kiichi became Yurie's guardian when she was a young girl, and he married her when she turned eighteen after keeping her locked up in his house for a decade and not permitting her to see the outside world, lest she be driven to leave him.
