Ishigami is a math teacher at high school. On his way to work, he passes by the usual cast of characters: homeless people living in shanties along the riverbank, an unemployed man sitting on a bench and dressed in his business suit, a widower who walks her three dogs, each of which has a collar with a separate color. He is able to deduce many details about each of these character's life situation, demonstrating his keen powers of observation and mathematical deduction. Instead of going directly to the high school where he works, Ishigami will take one "detour." In other words, he will stop at a certain lunch-box shop in order to see a particular woman with whom he is infatuated, Yusako Hanaoka, who works at the shop and who also happens to live in the apartment next door.

Yusako hardly pays any attention to Ishigami. It doesn't occur to her that Ishigami purposely comes to the shop to see her. It's not until Sanoko, the owner of the lunch box shop where she works, points this out that she becomes aware of the fact that Ishigami has a crush on her. At this point, though, Ishigami is mostly irrelevant to her life.

Yusako is a former nightclub hostess. It was at the invitation of her nightclub mama, Sanoko, that she works at the lunch box shop where she is now employed in a less "shady" and more socially acceptable line of work. Sanoko and her husband Yonazawa opened the shop together, it being a long-held dream of Sanoko to save enough money to start her own business. This works out well for Yusako, who would prefer to give up the nightclub job anyways since she has her daugther Misato, a junior high school student, to think of.

However, Misato is not the only reason that Yusako has decided to quit her job. She has an ex-husband, Togashi, whom she met while working at another nightclub different from the most recent one in which she worked. After Togashi lost his job, he became an alcholic and also became more abusive towards Yusako, which eventually culminated in them getting a divorce. Nonethess, this doesn't stop Togashi from going out of his way to look for Yusako and to harrass her for money. It is for this reason that Yusako changed her job once before, going to a different nightclub. It is for this reason also that she decided to take the job at the lunch box shop and move into her current apartment.

Unfortunately for Yusako, Togashi has found out about her new job and appears one day in her lunch box shop. When told by Yasuko to leave her alone, he threatens to seek out Misato, who is Yasuko's child from a marriage previous to that of the one between her and Togashi. Since Yasuko dreads the idea of Togashi being near Misato, Yasuko is willing to do anything to stop him from looking for her, including giving him money so that he'll go away. Unfortunately, rather than making him go away, giving him money only reinforces his "stalking" behavior and encourages him to make repeat visits.

Knocking on her door, one day, Togashi asks to come inside and talk with her. Yasuko adamantly refuses. Per his usual method of persuasion, he mentions how he might consider looking for Misato, which in turns prompts Yasuko to let him inside. What ensues is a dramatic and unexpected turn of events that departs from the usual exchanges that Yasuko has with her husband.

While Togashi is at the house, Misato returns home from school. Upon seeing Togashi, Misato is overcome with fear and escapes to her room in the back of the apartment. Togashi opens her sliding door to greet her, which infuriates Yasuko, who demands that he leave. As he is about to leave, Togashi suggests that Misato will be able to make money in a few short years' time, implying that Misato might end up a nightclub hostess or prostitute like her mother. When Yasuko warns him not to come back, he is smug in declaring that Yusako will always give in. As he reaches down to put on his shoes, Misato comes from behind him and smashes his head with a flower vase, knocking him to the ground. This takes Yasuko by surprise, and she is stares in shock, not certain what to do.