This is a mystery themed around the medical condition of depression. Okimura, the protagonist, is a clerk responsible for the medical records at a state-run hospital. One day, Katayama, a university friend he's not heard from for ten years who now works at the Ministry of Finance, contacts Okimura with a strange request. Katayama's sister is visiting a psychiatrist every week; since Katayama himself is out of the country, he wants Okimura to go with her. "Sister" (she detests her name, so only allows herself to be referred to in this way) is shut away at home, plowing her way through the literary classics all day. She refuses to speak, just nodding or shaking her head to indicate "yes" or "no." Okimura agrees to his friend's request on condition that he's paid a little something for his trouble, and starts going to the Katayama house every Saturday to take Sister to the clinic by train. She is a bit dopey as a result of all her medication, but physically seems well enough. The prescriptions she's given suggest to Okimura that she's being treated for depression (or could it just be diarrhea?). Okimura has a girlfriend (whose name we are also never told) with whom he has dinner and sex on Saturdays. The girlfriend is intrigued by the depressed woman's story and joins them on their visits to the clinic. This all comes to an end when Katayama returns to Japan after a year. In the meantime Okimura has started living with his girlfriend. She now begins to get depressed and Okimura has to take her to the psychiatrist every week. It seems that Sister and the girlfriend had been e-mailing one another without telling Okimura . . .