My name is Haibara Hikimori. In both name and reality, I am the man who will be the home security guard for the old Torimaru mansion. But before that happy day arrives, two sad events have occurred.

The first is that my cousin, Haibara Rena, is getting married this weekend. The second is that with her marriage, the owner of the old Torimaru mansion, Haibara Aizen, will be joining us. My maternal grandfather suggested gathering all the direct descendants of the Haibara family at the old Torimaru mansion. I don't know if he's a local celebrity or an old family member, but the head of the family has an old, bad kinship mentality where anything he says is absolute, so the Haibara family has decided to stay at the old Torimaru mansion for a few days as a homestay guest.

There are seven people staying as homestay guests. The senile old man who suggested it. Haibara Aizen. My mother's younger sister, Haibara Shiho. Shiho's eldest daughter, Haibara Rena. Her second daughter, Haibara Aya. Her third daughter, Haibara Narumi. Aizen's adopted daughter, Haibara Shieri. Haibara Reina's fiancé, Miyano Kouji.
The culprit is among them. I swear by the name of my paternal grandfather.

[From Official website]