流水亭に流れる殺意

Conan, Kogoro, and Ran are out to eat at a fancy restaurant with a stream that little boats carrying food sail to the rooms they are supposed to deliver the food to. After a while three men enter the restaurant, then after eating a woman screams and Kogoro, Conan, and Ran rush to the frightened women, pointing at a dead person. How was the murder done? And why was the victim in the room he wasn't supposed to be in?