The protagonist, Mihara Suzu, a student at Akarinkai Senior High School (commonly called “Akarin”), was heading towards school with her childhood friends Otonari Kanade and Shizuma Souta, along with her best friend Matsuri Ayame, as per usual. When they arrived at the front gates, they hear a certain story from Akarin High’s student council president, Aoba Taiga:

“The truth is, one of the students has gone missing.”

According to Taiga’s story, during the previous night, an Akarin High School student boy went missing after acting like a different person; behaving violently while he was playing with his friends. Though the story worried them, the bell signalling the beginning of the first class forced them to end the conversation.

The next day, what Suzu and her friends saw in the schoolyard was the boy who went missing. He had a bizarre appearance and was crucified.

A strange incident that plagued the peaceful school.
From there on, a mystery interwoven by love and madness begins. How will it end?

[Translation of the official website's story summary.]