A small package arrives at the detective agency from an overseas journalist who has worked with Jinguji before. Inside is a single key — no instructions, no context, nothing to indicate what it opens or why it needed to be sent across an ocean. What the journalist left behind, whether deliberately or not, is a puzzle that only Jinguji is positioned to unravel. Finding the meaning of the key — what it was meant to open, and why someone took the trouble — is the case.