Kobato and Osanai made a pact to lead a quiet life as regular students upon entering high school. However, soon after, a handbag mysteriously disappears at the school, and Kobato is asked to help solve the case.

To top off their summer sweets tour around the city, Osanai treats Kobato to a Tropical Parfait as thanks for rescuing her. Along with this delicious treat, Kobato is also served the whole truth about Osanai's kidnapping.

After a long conversation, Osanai and Kobato come to the conclusion that their agreement has failed – and with it, their friendship. From then on, they go their separate ways.

After starting to get involved with Osanai, Itsukaichi investigates a possible string of fires in the city.

Kobato explains to Osanai how he arrived at his conclusion about the arsonist’s identity.

Kobato and Osanai finally find out the whole truth and get help from an old acquaintance.