By the decision of the Sonozaki family, Sonozaki Shion has been "imprisoned" at St. Lucia Academy, an all-girls boarding school. One day, a male teacher is found dead in the school's swimming pool, and Shion's classmate Kousaka Mizuho, who was the first one to discover the body, is summoned to the board chairman's office, but Mizuho refuses to discuss the details of the incident. Shion hears a rumor that Mizuho's own grandmother is trying to kill her and that she entered the school to escape the danger, and uses her French homework as an excuse to try to make contact with Mizuho, who refuses to open up to anyone.