Brush of Her Hand. Book 1

This riddle cannot have a solution. And yet it does…

She came to the legendary detective Kiyoshi Mitarai — this strange woman named Kaio Ninomiya. However, he has long been away from home, traveling through different countries. She was received by the detective's friend, writer Kazumi Ishioka. The woman's request couldn't be more absurd: Ishioka must travel with her to a distant wilderness to find there the buried… brush of her own hand. At the same time, both of her hands are intact. Surprising himself, the writer agrees.
And now they are in a mysterious mountain village at the quirky and grim hotel "Ryugatê." But they will have to put their case on hold: an impossible murder occurs at the hotel. In a locked room, well-visible with tightly shut panoramic windows to all walls, a woman playing music on a traditional Japanese instrument — koto, dies from a shot to the head. And at that moment, the room is engulfed in flames…
This riddle is just for Mitarai. But he is not in Japan, and contacting him can only be done in writing…