Case Closed (1996)

Season 6

Episode 27: The Murder Floating in Ryusuitei

Mouri, Ran, and Conan are at a fancy traditional Japanese restaurant where food is delivered on a small computer controlled boat that navigates over a running stream around the restaurant. Later, a murder occurs and Conan realizes the only way it could have been possible was for a person to be alone, and to use the stream to access the room. Conan later reveals that the murderer hid in the boats and came out to murder the victim. The culprit explained how he was blackmailed by the victim who demanded a large payment from him.