Astronauts: Sirius - Hyakki Ryouran no Yakata

Aoyama Yoshito is a university student in the folklore studies faculty.
When Yoshito was chasing after his faculty lecturer he ended up in a particular area, got himself lost in a mountain far from human habitation and finally arrived at a certain Western-style mansion.

This mansion was owned by the immensely wealthy family of old nobility, the Mikado clan.

Yoshito was permitted to stay at the mansion as a guest and forced to participate in the "Eigu rite" which would decide the next head of the Mikado clan.

The three beautiful but mysterious Mikado sisters, the assembled candidates of the rite invited by the Mikados as well as the mansion's inhabitants, the previous Mikado clan head Heizou and the current head Fumika.

What fate awaits Yoshito in this mansion where the desires of men and women intermingle?