The autocratic president of a vast Tokyo hotel is found stabbed inside a double locked room. His beautiful secretary becomes a suspect, but she has an ironclad alibi: she spent the night with detective Hiraga. When she is later found dead in Fukuoka, Hiraga pursues the truth linking the locked-room device to the distant death.

Before a double locked room and an unbreakable alibi, a detective's determination enters the darkness of a vast hotel.