Based on the Drama CD series by Broccoli.

Who killed Blue Robin?

There is a mansion in the town where both of the protagonists (Rei & Akira) grew up in that is known as the Manor of Misfortune. On the day when Rei visits the place while hired as a private detective, a murder occurs. Then enters Akira, who now works as a police investigator, to lead to one of the worst reunions of old friends.

After basically avoiding each other for years after a certain incident, it’s time to slowly put away their sad past to focus on solving the case…

[edited from Koorin's blog]