After the body of Otaro's former partner Kyujiro is found, Otaro is found dead in a locked room at the Hatobue Lodge. It becomes clear that Otaro was one of two culprits wearing Byun masks responsible for the robbery and murder that happened seven years ago. It also comes to light that Manager Yuri Asako's daughter Miona was the witness in the case. The police believe Otaro died in his room accidentally after killing Kyujiro to keep him quiet, but... While investigating Otaro's room, Conan closes in on the truth...

Kogoro, Conan, and Ran arrive at a restaurant with a sitting room looking out on a river. The watermill upstream catches Conan's eye. Later, they meet Teito University students Juri, Maria, and Yuko, who ask Kogoro for feedback on the photos they've been taking to submit to a photo contest. All three leave the restaurant for one reason or another, but only Maria and Juri return. Just as they are wondering where Yuko went, they see a body floating down the river amidst the blood red water...