Kagiana Satsujin Jiken

A strange request comes to Scotland Yard. Client Gilles Willcock receives a murder notice that's bound to happen at midnight. He calls out a detective, and seven people at the mansion wait nervously for escalation. Gilles locks him up at the second floor while detective hides in waiting at the first floor. At midnight gunshots are heard on the second floor, and a murder is seen through a keyhole. When detective finds the key and enters, there is no body. Mansion stuff and interior hide the keys to the mystery.

[From vndbreview]