4 ideas
Mystery-club members Hamura Yuzuru and Akechi Kyosuke join a film club's summer retreat at Villa Violet, alongside the student detective Hiruko Kenzaki. A bioterror attack at a nearby rock festival turns crowds into zombies and traps the survivors inside, and then guests start dying. The killer turns out to be avenging a friend who was sexually exploited at the club's previous retreat.
A fantastic element does not break a fair-play puzzle if its rules are stated clearly and never bent. The zombies bite, infect after a delay, can't operate complex mechanisms and are drawn to living people, so each rule becomes a premise the reader can reason from. The impossible thing turns into a new kind of physical evidence rather than an excuse for anything.
When a victim can be doomed hours before they die or turn, the time of the killing and the time of death become different moments. The killer and the detective both work in that gap: alibis, locked doors and who was where must be judged against when the fatal exposure happened, not when the body was found. A lag between cause and visible effect makes normal timeline reasoning fail.
The story sets up Akechi as its Holmes and then removes him early, turning genre expectations into misdirection. This passes the reasoning to a less obvious detective and tells the reader that no role guarantees survival or authority. Readers who trust the assigned roles of a form are the easiest to mislead.