The protagonist suddenly received a message from her boyfriend Aida Takumi- whom she had been unable to get in contact with since spring of this year- to "meet me in Classroom 901".

However, after dashing to that classroom, the person waiting for her there wasn't Aida, but her underclassman Takazaki Yuki.

He gives the protagonist a mysterious smile as she searches for any sign of Aida's presence...

[Translation of official website story summary.]