So that people know what they're getting into... here's a summary. Feel free to replace this when someone comes along with a good one.

The story starts with an entry on a suicide group's website. Five people plan to suicide together, and 'Valkyrie' will be providing food and transportation. The group meets at a station somewhere, and Valkyrie is the last to arrive. When Valkyrie does arrive, she seems to be a cheerful woman, not the sort to commit suicide. Anyway, the food she brought was drugged, and the would-be suiciders wake up in bunny suits on a mountain. What sort of game will they play?

In a Meiji Era, there's a small church. The church, however, had only one sister that carries the duties. She was Satanister. Satanister is not your ordinary sister. She uses violence to settle her problems. As her name suggest, she's a sister close to the Satan.