Case Closed (Detective Conan): 1x161

The Murder Floating in Ryusuitei

Kogoro, 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.