Professor Agasa takes the Detective Boys with him to an onsen (warm spring) to fix a product the propietor purchased from him. At the onsen, the kids are introduced to four cast members filming a drama about a hitman. Since the bath is crowded, the k... (more)ids decide to bathe in the early morning. As they head to the bath, Ai and Ayumi discover the Ganji Tetsuyama, the scriptwriter, inside the bath's women section, dead.