探偵・癸生川凌介事件譚「五月雨は鈍色の調べ」

The case is set in Marihama-dai during the rainy season, the first heavy rain in eight years, in a dull rain pattern that spreads as if to take away the color of the town.
The story begins with a client visiting the "Kibukawa Detective Agency.

This is the Kibukawa Detective Agency, isn't it?
I believe there is an assistant detective here named Sagishima-san.
No, no, not a girl like you...there was a young man named Sagishima, wasn't there? There was a young man named Sagishima..."

The client said he had come to fulfill a "promise made eight years ago" to a young man named Sagishima, who used to work at the detective agency.
As heavy rain pelted the window, Izuna Sagishima, the current assistant detective, began to listen to the client's story in order to take over the case.

He is told about a case he took on eight years ago, which the young man, who used to work at the detective agency side by side with Ryosuke Kibukawa, took on.