In contemporary Tokyo’s Himonya district, twenty-seven-year-old elementary school teacher Kaede finds herself surrounded by everyday mysteries she cannot solve on her own. For guidance, she turns to her grandfather, a retired school principal with Lewy Body Dementia who was once a celebrated member of the Waseda Mystery Club. Although his memory is fading and he struggles with hallucinations, the grandfather retains a remarkably sharp, lucid mind when presented with a genuine puzzle. Together, the pair adopt the practice of weaving stories – constructing and testing different narrative explanations until they close in on the truth behind each case.

The narrative is structured as six interlinked mysteries that escalate in complexity. Early cases include a locked-room murder at an izakaya, a teacher who vanishes from a school swimming pool, and a classroom ghost that appears during an English lesson. As Kaede brings each problem to her grandfather, the solutions reveal not only the mechanics of the crime but also the hidden grief, shame, and compassion of the people involved. Recurring supporting figures include Iwata, Kaede’s well-meaning colleague, and his friend Shiki, a blunt and magnetic actor who challenges Kaede’s deductive instincts and becomes a potential romantic interest. Beneath these episodes, a darker thread emerges: a stalker begins to close in on Kaede, forcing her to confront a traumatic event from her past involving her mother.

The grandfather’s condition is depicted with matter-of-fact tenderness. His lucid intervals are temporary and do not halt his decline, but the act of solving puzzles allows Kaede to remain connected to the teacher and hero he once was. The mysteries themselves are fair-play puzzles rooted in the Golden Age tradition, with small observations and offhand remarks serving as crucial clues. The tone remains compassionate, focusing on understanding human fallibility rather than punishment. The book culminates in a final confrontation that tests the grandfather’s waning faculties and forces Kaede to reckon with the full weight of what she is about to lose.