Answer

"A detective story must have as its main interest the unravelling of a mystery; a mystery whose elements are clearly presented to the reader at an early stage in the proceedings, and whose nature is such as to arouse curiosity, a curiosity which is gratified at the end."

"The Ten Commandments for Detective Fiction"

I'll be doing my impression of a broken record here, but I am of the opinion one can do a lot within the mystery fiction genre, and that for example, supernatural or fantasy elements do not, by default, threaten the internal integrity of the genre, in the same sense that realism does not automatically mean a mystery story is actually good or fair.  So what if there is an experimental drug that can turn a sixteen year old boy into a child? Detective Conan's core premise may be based on something out of a fantasy novel, but there have also been various stories within the series that present a completely fair and expertly written mystery plot that utilize the whole fact such a drug exists. A story about actual ghosts that can attack real-life people? Invented Inference (In/Spectre) is easily one of the more entertaining and exciting mystery novels I've read this year, one that at least dares to focus on what makes a mystery plot interesting and build a story around that core idea, rather than just using the familiar set forms of locked rooms murders or anything like that. Actual prophecies destined to come true, magic watches that show you to the exact moment of death of any person, a murder mystery set in Alice's Wonderland, robotic cats with technology from the future and a 5000 light year road trip through space: this year alone I've gone through heaps of great mystery stories that utilize the supernatural in one way or another, and all of them were interesting puzzle plot mysteries that played the game square and fair. What's important for a good puzzle plot mystery story is having a consistent internal logic, not boring realism!

Anyway, this got me thinking about what a "mystery" in a mystery/detective story means to me. Because to be completely honest: most of the 'conventional' mystery stories don't even try to something truly original with what a mystery could be, and keep to the familiar murders and other crimes. We have the familiar whodunnits, howdunnits, whydunnits, howcatchems and the occasional whatthehell (where what appears to be a normal story is revealed to much more, putting previous events in a different context). They may provide an original take on for example the locked room mystery, but they don't attempt at taking one single step back, at examining what a mystery could also entail. To me, a mystery or detective story is not about murders or crime. Like Knox said, it's about a mystery: an unanswered event. The question of who murdered Roger Ackroyd may be such a mystery, just like all the familiar tropes like locked room murders, perfect alibis, whodunnits and whatever comes to mind right away, but the rather mundane question of who someone managed to make hot cocoa with only three mugs and one teaspoon is also a valid mystery. I am not a particular fan of the everyday life mystery subgenre, but I'll be the first to admit I've seen some great stories involving mysteries about seemingly mundane, but still curious and alluring problems, like the problem of the food stalls at a summer festival all returning change in 50 yen coins instead of 100 yen coins.

What can a mystery also be? Some months ago, I read Astra Lost in Space and the first half of that science fiction mystery series involved a type of mystery you never see in 'conventional' mystery stories. A group of space-stranded students try to make their way back to their home planet with their spaceship the Astra. They have to make pitstops at several unknown planets due to the length of their trip and some of these planets house threats which only manifest when it's almost too late. Astra Lost in Space really shines during these moments, as these 'creeping' dangers on the various planets are always well-hinted and foreshadowed before they are actually shown to the reader. On the second planet they encounter for example, the assumption that everything is the same as back home almost leads to fatal conclusions, but both the team, and the reader, could've foreseen the reveal, as it's properly hinted in the narrative from the moment they land on the planet. Whether it's the question of how Lt. Fukuie is going to uncover the ingenious murderer even though the reader already knows what the murderer did in detail, the discovery of a 50.000 year old corpse on the moon, or just the question of why someone decided to litter and not bring their food tray back inside the canteen: a mystery (in a mystery/detective story) can involve anything, as long it's presented as a genuine curious problem and the solution is based on properly presented clues and internally consistent logic.

By the way, I keep saying I'll write a review of Liar Game one day, which too is a fantastic mystery series that uses a completely original premise, revolving around a series of gamble games like Minority Rule (where you want to be voting for the minority) or the Contraband Game (where two teams have to smuggle money to from one room to another, and each team has to play border patrol) that appear to be games of pure chance, but which can be 'rigged' by both pure logic and psychological warfare. It's a great example of what the mystery genre can also offer. One day, I'll really write the review. But not this year.

Earlier this week, the first trailer for the 2020 Detective Conan film The Scarlet Bullet was released, and not surprisingly, it seems to continue the trend of the last few years to be somewhat action-focused. These films have always been more action-focused than the original comics for obvious reasons, but looking back, I have to admit there have also been some great action scenes in these films that are also properly build on a mystery story model. Usually, these scenes involve Conan having to escape some imminent danger, and he eventually manages so by cleverly using the tools available to him. So you have the mystery (how is he going to escape?) and the solution (earlier shots of what's available to Conan as hints, and there's of course internal logic). 2002's The Phantom of Baker Street has a grand climax scene for example where Conan has to survive a very imminent crash of the steam train he's riding into the station. Conan's given verbal and visual clues, and in the end, he comes up with a clever way to not get crushed into a pulp. I'll be the first to admit that the viewer is given very little time to consider the problem themselves, but it's without a doubt a fair puzzle plot. But let's take for example 2008's Iron Man, the movie that properly kicked off Marvel's Cinematic Universe. In the final act, Tony Stark is having quite some trouble fighting off Iron Monger in his own Iron Man suit, until a certain event that helps give Tony an edge over the larger/more powerful Iron Monger suit. This too follows the cycle of having a problem (how's Tony going to win?) presented, and proper build-up/hinting (that one thing happening to Tony earlier himself too) and internal logic. Now I'm not going to say Iron Man's a detective movie because of that one scene, but I do think a mystery story can be much more varied than a lot of people seem to think, because the core values of the genre can be applied in so many ways. While it does need proper set-up to be considered a mystery/detective story, I do think anything can be a mystery.

Anyway, I'd be interested to hear some thoughts about how others look at the concept of "mystery" in the genre and perhaps hear about some personal favorites of examples of not-so-likely mystery fiction.