Right before summer vacation, a "forbidden game" is secretly played at a high school. After some students take part in it, their ordinary days begin to change, and before long, a "certain incident" occurs. Taking the situation seriously, the school brings in a counselor, Koharu, to provide mental care for the students who witnessed the incident. However, one of the students who played the "game" turns out to be Koharu's sister, Nanami. One by one, the students are drawn into bizarre phenomena.