The "Kailoukan", a strange building surrounded by the sea on all sides. Inside a mansion closed underwater, the curtain rises on a tragedy. Hello, everyone. My name is Ikurumi, a game scenario writer. Do you know the previous work "Masked Fantasy Murder Case"? This time, I would like to introduce an incident that happened in a strange building called "Kairokan" six months after that incident. It is said that it was built by the former chairman of a famous architectural company as a stranger. This story begins from the place where we are invited to the inauguration event of the mansion. 7 invited guests who visited. The owner of the mansion who will not appear forever. And... A tragedy that happens in a mansion closed in the sea. Who is the culprit? What's the motive? And what is the secret hidden in this building? The truth is, in the dark sea...