Yoshioka, an experienced detective, investigates the murder of an unknown woman in a red dress. She was drowned on the Tokyo waterfront, but an autopsy reveals that her stomach is full of seawater. Moreover, all the clues he finds relate to himself: A button found at the murder scene matches one that is missing from his own coat, and fingerprints found match his own. Yoshioka realizes that the only viable suspect is himself; but he doesn't remember a thing.