Following troublesome middle school experiences, where Osanai's calculating vengefulness and your compulsive detective work both caused problems, you've made a "Petit Bourgeois" pact, a mutual agreement to keep each other in check and pursue ordinary, peaceful lives.

Today, that peaceful and ordinary existence is threatened by something as simple as unexpected rain and a forgotten umbrella, leaving Osanai stranded at school among crowds of chattering students she'd rather avoid.

Osanai is from the Shoushimin Series and you take Kobato's place in the story.