Touma Kojirou's Detective File -Murder at the Opera House-

Touma Kojirou was just a regular, ordinary detective, investigating suspicions of adultery and finding lost pets until one day, his little sister brings her friend in as a client. As her theater troupe prepares for their performance of “Phantom of the Opera”, a real phantom threatens to end their production!

Will you be able to uncover the identity of this masked assailant before its too late?! Or will you be too busy banging every member of the cast?! Find out, when the curtain rises!

[From MangaGamer]