Masayoshi Negishi is one of four male students at Nankai Academy who investigates Professor Yuko's murder after police dismiss it as an accident. He uncovers clues by reviewing hidden camera footage placed throughout the school, which captures interactions of female students suspected of involvement in the professor's death.

Using evidence from the professor's schedule book, Masayoshi confronts the suspects. He directly participates in punishing Sae Ichijyo, instructing her to change clothing and taking her to a bathroom stall for further discipline. He similarly confronts Kiyoka Enjyoji at her estate, where parallel punitive measures unfold.

Throughout the investigation into Professor Yuko's death, Masayoshi continues his activities as part of the group's collective effort. They employ surveillance and direct confrontations with female suspects noted by the professor, though the inquiry's conclusions grow increasingly complex and unclear.