Tantei Kibukawa Ryousuke Jikentan 'Tsuikousaku Jiken'

This case is a series of murders that have been occurring in and around Marihama-dai.
The victims are not connected to each other, and Detective Otonari, who sees that the situation is grave and has no clue how to solve it, requests you to help him solve the case.
The story begins with the assistant detective, Izuna Sagishima, who is working with the police to investigate the case.

Meanwhile, a detective agency is forced to do a half-assed job on the detective's behalf.
Masao Ikurumi, a scenario writer, is asked by a company to search for a person.
The woman he was investigating, Yumiko Hamakawa, was a contract employee working in the secretarial department.
After a month of work, she suddenly disappeared...

Izuna and Ikurumi. They encounter two incidents. What is the truth that awaits them at the end of it all?