Case Closed (1996)

Season 6

Episode 21: The Key in the Water Locked Room Case

Detective Mori, Ran, and Conan pay a visit to an old woman. She decides to take them to her grandson, but he's found dead when they get into the house. The bathroom door is locked however. How could this have happened? Rather than use detective Mouri for the case, Conan solves this one as himself.