Girl's High School.

Six young women, including Katsuki Takahashi, hold a high school reunion, but their high school has been shut down. The women are happy to see each other again. Suddenly, a shot rings out and one person dies. They receive a text message from Natumi Shirakawa who died 7 years ago. The text message asks "who is the criminal?" To figure out the identity of the perpetrator they look back at what happened 7 years ago.