Takashi Shimizu's newest film is about a mysterious cassette tape containing a melody. People who hear this melody gradually begin to hum it themselves and become involved in a mysterious incident. What is the mystery behind this melody ?

During the summer vacation period, temporary teacher Honoka Kimijima is in charge of a supplementary class. One day, a female student suddenly jumps off from the school rooftop and dies in front of Honoka Kimijima. Then she notices a mystery student who is not supposed to be there.

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.