4 ideas
A textile company magnate is found shot, and suspicion settles on a person whose account places them far from the scene, apparently confirmed by fixed railway schedules and independent witnesses. Inspector Onitsura does not look for a new suspect; he takes the unbreakable alibi itself as the object of investigation and rebuilds the suspect's movements train by train until he finds the one gap in the schedule that makes the killing possible.
Ordinary innocent movements are loose and badly documented, while a manufactured alibi is unusually precise, well witnessed, and tied to external fixed records. When a suspect's alibi is too complete, that completeness is evidence it was designed, and it tells the investigator where the deception is hidden.
Break the claimed itinerary into discrete segments, each with a departure, an arrival, and a mode of travel. Test every segment against the real schedules for alternative routes, connections, and vehicles the suspect could have used instead. The alibi fails at the single leg where a faster or unrecorded path makes the stated timeline unnecessary, even if every other leg checks out.
Onitsura solves the case through slow, repetitive checking: consulting schedules, re-interviewing witnesses, and physically retracing routes, rather than through a sudden flash of genius. This treats detection as exhaustive search over a constrained possibility space, where persistence in closing off options produces the solution that intuition would miss.