The days when mail carriers were called postmen are a thing of the past. Kasugaura Haru, who works at the counter of Yotsubazaka Post Office, loved her strict but kind senior, Rokuro. However, one day, an employee from the Postal Administration Office visits Yotsubazaka Post Office and begins to speak as if suspecting Rokuro, saying there has been an accusation of undelivered mail under anonymity. Thus begins a gentle postal mystery set in the world of ‘postmen’!