As the police continue their investigation, they discover a plastic tea bottle filled with water in the dressing room, match cinders floating on the water, and Tetsuyama's silver-gemed ring. It is also revealed that all the suspects have been seen in the hotel after the sauna was closed disproving their previous theory.

Kogoro receives a letter from an unknown person. Telling them to go to the Koura Inn to catch a murderer. Following the letter Kogoro, Conan, and Ran goes to Koura Inn in the Gunma prefecture. On the way they met two people named Rokuro Tokubi and Kazuki Araiwa. They ask to stop at a river on the way. It appears eleven years ago near the inn, an elementary school student named Tatsuhiko Numayama got swept by a flood and died. Could this have been the murderer? Then a man comes a kicks over the grave (rocks) made by the deceased friends. This is shown to be the father of the deceased. They go to the inn and find out that the owner of the inn is the father as well. That night Ran, Kogoro, and Conan go back to the river because Ran lost her phone. Once Ran finds it, she sees a silhouette of a Kappa. The next morning, Banzo Numayuma is found dead in the attic, drowned in the reservoir water that his son's body was found.