4 ideas
When the culprit's identity is revealed before any evidence is assembled, the puzzle shifts from 'who did it' to 'how can this be shown to someone who does not accept the source.' The detective has to work backward from a certain conclusion and build a chain of admissible, observable facts that reaches it. The revelation only counts once that chain exists, because testimony from a spirit persuades no court and no skeptic.
Insight that looks supernatural can be ordinary deduction whose premises have been hidden: small physical cues, behavioral tells and cold-reading techniques, presented as a vision. Once the 'oracle' is read this way, each correct revelation becomes a question about which clues the speaker noticed and chose not to disclose. That question can be checked. The ability to arrive without showing the route is what creates the impression of the uncanny.
A source believed to be never wrong stops being examined. Observers then focus on confirming its outputs and stop asking what it wants or whom it is steering. This blind spot runs in both directions: the partner who seems to be only a validator can use the relationship to steer the oracle, and the oracle can use its reputation to steer the partner. Perceived reliability therefore becomes a tool for manipulation, not a safeguard against it.
Mystery novelist Kōgetsu Shirō, who advises the police, teams up with the young medium Jōzuka Hisui through a series of murder cases, including a separate serial strangling of young women. In each case Hisui supplies the answer through 'spirit' insight and Kōgetsu builds the logical proof. The final section reverses the setup: Hisui's powers were staged deduction and cold reading, Kōgetsu is the serial killer, and she has used her persona to draw him out and prove his guilt by the same logical method the book's fair-play rules require of any detective.