It's almost like eight out of ten books discussed here will feature supernatural or science-fiction elements, whether it's "just" the suggestion of, or actual supernatural powers. And in general, the latter category actually seems to have an even better track record...
Kuramochi Yuika, a receptionist at a department store, approaches her friend and mystery writer Kougetsu Shirou to accompany her on her visit to a spirit medium. Yuika's been seeing weird visions of a weeping woman and a fortune teller has advised her to go see the spirit medium Jouzuka Hisui, as she's supposed to be the real deal. Kougetsu is not only a mystery writer, but also occasionally assists the police in their investigations as a criminal profiler, so he's a bit sceptical at first, but the way in which Hisui manages to sense both Yuika and Kougetsu's professions from their aura is astounding, giving his first moments of doubt. The beautiful spirit medium senses some lurking danger from Yuika's aura, and wants to take a look at Yuika's apartment. They agree to meet a few days later, but Yuika doesn't appear at the station as promised and when Jisui and Kougetsu make their way to Yuika's address on their own, they find Yuika lying dead in her apartment. The crime scene suggests someone pushed Yuika, causing her to hit her head on the table, killing her. The initial police investigation focuses on two scenarios, the murder being either the work of a notorious neighborhood burglar or one of Yuika's personal acquaintances with stalker tendencies, but Hisui reveals she can in fact channel Yuika's spirit in her dying moments. Hisui's vision is vague, but it does reveal the culprit was a woman, who was apparently looking for something as Yuika's consciousness faded away forever. Kougetsu is now however facing a problem: the police isn't likely to believe Hisui's vision, so now he has to figure out who killed his friend based on Hisui's vision and also find supporting proof, as the vision alone won't have any value when talking to the police. After the duo manages to solve Yuika's murder, Kougetsu and Hisui run into more mysterious cases that require Hisui's unique powers, but Hisui's also foreseen her own imminent and only Kougetsu can prevent her death in Aizawa Sako's Medium - Kourei Tantei Jouzuka Hisui ("Medium - The Medium Detective Jouzuka Hisui" 2019).
Medium - Kourei Tantei Jouzuka Hisui has been one of the more prominent releases of 2019: it managed to rank in several of the annual top ten mystery rankings organized by Japanese publishers and in the period between me purchasing this book and actually reading it, it also won the 2020 Honkaku Mystery Award, which usually means fans of puzzle plot mysteries will enjoy the work. Personally, it was the premise that really manage to hook me in. As you may have noticed, some of the best, fair-play mystery fiction I've read these last few years feature supernatural elements, like the murder-fable short story collection Mukashi Mukashi Aru Tokoro ni, Shitai ga Arimashita, the mega-hit Shijinsou no Satsujin and its sequels/adaptations, Kobayashi's wonderful mysteries based on Alice in Wonderland and E.T.A. Hoffman's works, the time-travelling mystery Jikuu Ryokousha no Sunadokei and the yokai mystery Invented Inference. Medium - Kourei Tantei Jouzuka Hisui obviously features a spirit medium and while often mysteries featuring spirit mediums feature frauds, you also have examples of great mystery fiction featuring real spirit mediums and where their powers serve as a source for original mystery plots like in the 3DS game Gyakuten Saiban 6/Phoenix Wright: Ace Attorney - Spirit of Justice.
Medium - Kourei Tantei Jouzuka Hisui follows an interlinked short story collection format and the first story, The Weeping Woman Murder, serves as a good introduction to show how Hisui's visions can still work in a fair-play mystery plot. It's established right away that Hisui's powers definitely have their limitations. For example, Hisui can sense emotions and feelings like guilt from people's aura, and also sense whether one's aura is being under attack by someone else, but obviously, she can't just go to the police and say that this or that person 'feels' suspicious. But her powers do allow Kougetsu to deduce who isn't the murderer, allowing him to focus on the real culprit and finding tangible evidence. Basically, the stories revolve around Kougetsu being put on the correct rail right from the start thanks to Hisui's powers, but it's Kougetsu who has to reverse-engineer her visions and find real proof and come up with a supporting line of reasoning. It reminds of Morikawa Tomoki's Snow White, in which a magic mirror would show the answer to a mystery, but where the detective still had to think of a convincing deduction herself, because people would not believe her if she'd just say the answer. Invented Inference falls in the same family too, where Kotoko invents inferences to lead to the conclusion she already knows beforehand because ghosts and other supernatural beings help her. In The Weeping Woman Murder, fragments of Hisui's vision and what she senses from auras help Kougetsu on the right path straight away. Hisui even reveals she knows the murderer's a woman the moment they discover the body, but ultimately, it's Kougetsu who has to figure out the meaning of the actions of the woman seen in the vision, its implications and who the murderer is. And on top of that, he has to come up with a line of reasoning that supports his theory, a line of reasoning that doesn't rely on Hisui's powers, because obviously, the police wouldn't believe any of that. The result is a truly interesting story that shows what makes a mystery story so fun, as it plays with the notion of what a mystery is and also with the idea that a logical deduction doesn't need to be true, only convincing and entertaining.
In the second story, The Murder in the Water Mirror Manor, Kougetsu and Hisui are invited to a small barbecue party at the Water Mirror Manor, the lakeside second home of the celebrated mystery writer Kurogoshi Atsushi. Having heard about Hisui from Kougetsu, Kurogoshi hopes Hisui can find out whether his home is really haunted like his family thinks and if so, he hopes she can do something about it. But this is kept hush-hush, and Kougetsu introduces Hisui as his Plus One to other mystery authors and editors invited to the party. Kougetsu and Hisui are to stay at the Water Mirror Manor that night, as do three other guests. The detective duo stay up most of the night in the living room that connects the manor's two wings to see if there's any spectral activity and it's during this period that the three other guests all pass by the living room at one time or another, as the bathroom in the wing with the guest rooms is curently broken. The following morning, Kurogoshi is found to have been bludgeoned to death in his study in the other wing. Kougetsu and Hisui were in the living room during the estimated time of death, so the murderer must be one of the three persons who passed by the living room, but... Hisui can tell right away who the murderer is based on their auras. The problem remains the same however: how is Kougetsu going to prove that? A strange vision seen by Hisui serves as a clue to what happened, but can Kougetsu also find supporting evidence for that?
Unlike the first story, this story straightout reveals who the murderer is by name, which might make you think it'll be easy, but oh-no! This is a very tricky story: the deduction chain that revolves around Hisui's vision is something you'd expect to see in Queen's work, revolving around the actions taken by several characters and the implications of those actions. The story makes good use of the backstory of the manor, but what really seals the deal is that even if you figure out what Hisui's vague visions actually mean, you still have a lot to do, as Kougetsu's ultimate goal is always to come up with a line of reasoning supported by the physical evidence! So even if you've "done" Hisui's part of the mystery (identifying what the vision was about and its implications), you still have to figure out a seperate line of deduction that will lead to the same conclusion, a line of deduction that will convince the police to act. The clewing for that is pretty clever too: it's not super surprising, but I have to admit I don't think I've come across this particular version of the idea before, though older variations are fairly common.
In The Serial Murder Case of the Female High School Student Strangulations, Kougetsu is having a signing event, when he's approached by a fan. In the last year or so, several of Natsuki's schoolmates have been strangled to death. The police investigation has led to no results at all, so Natsuki hopes that Kougetsu can help find the killer. Because of his track record, the police allow him and his "assistant" Hisui to join the investigation and start snooping around the high school. Hisui soon senses the police is on the right track, but before they can act, another victim falls... Fearing the culprit may be feeling cornered, they try to act before more victims follow. This story is a bit simpler in set-up compared to the previous stories: Hisui's powers don't really lend them well for this type of case, so while she's able to point the police investigation to the correct direction early on, it's actually Kougetsu who has to do a lot of the heavy lifting himself this time, using the few hints Hisui manages to convey to him to figure out who's the serial strangler. The focus lies a bit more on the interpretation of physical evidence this time, but the climax of the story really works well a supernatural-themed mystery.
The final story VS Eliminator Kougetsu and Hisui are asked to find another serial killer: several women have fallen victim to this murderer for the last few years, who has been mentioned a few times in previous stories as well as in the story intermezzo parts. It's hard to explain this story without giving too much away, but this story is excellent and works wonderful as the conclusion to the whole book, and it elevates the work to a genuine must-read of the genre. Early on in this story, we find out that Hisui's powers actually don't really mesh well with this type of case: the previous stories established that Hisui usually needs to be physically at the murder scene to be able to have her visions, or she must come in close contact with suspects to feel their aura. It's because of this that Kougetsu doesn't feel much for involving Hisui in this case, as her powers may attract the attention of the killer and in any case, her powers don't really apply here, but Hisui's still determined to find the serial murderer. What follows is a fantastic tour-de-force in deduction that show off Hisui's real powers in full might: whereas the previous stories seemed to prove the limits of Hisui's gift, VS Eliminator actually turns everything around: in a mesmerizing super-flashback scene we see how all those moments that only seemed to prove the limitations of Hisui's supernatural powers and all the other events actually all line up in a single arrow, proving in fact how utterly overpowered Hisui's skills really are. This story really invites you to read the book all over again from the start, as many scenes suddenly take on a different meaning, and you see how carefully author Aizawa has been planning this climax, with Hisui's power being proven to be much more than the reader is first led to believe. Each previous story turns out to be foreshadowing this conclusion with proper clewing, while also telling a good mystery tale on its own, so it's like each story was telling two stories at the same time: one "front" story, and a "back" story that is only revealed in this final tale. But while Hisui's newly revealed powers ultimately allow them to catch the serial killer, the book still holds on firmly to the fact that this is a fair-play mystery novel, built on brilliant deductions and actual physical proof.
Medium - Kourei Tantei Jouzuka Hisui is definitely a masterpiece of the genre, one that really shows off how supernatural elements do not hinder a mystery plot, but can actually greatly open up possibilities when used well. The individual short stories are entertaining on their own, showing off several ways in which pure logic and supernatural elements can work together, but it's really the final chapter that makes this novel more than 'just' a memorable mystery. This is an absolute must-read, and I wish most mystery novels would spend at least half the amount of planning effort Aizawa poured into this novel: the chains of deductions displayed in this novel are fantastic and the pay-off at both the micro and a macro level of this novel is something indeed very few novels manage to accomplish.
Original Japanese title(s): 相沢沙呼 『Medium 霊媒探偵城塚翡翠』