The narrative begins with the protagonist, Aida Yuji, a high school senior facing academic struggles, specifically a failing grade that necessitates a make-up test. While contemplating his predicament on a rainy afternoon, he encounters a shy, twin-tailed underclassman who has forgotten her umbrella. In a moment of impulsive chivalry, Yuji offers her his umbrella, running out into the downpour himself, a decision he later regrets due to embarrassment. This encounter marks his first meeting with Fujisaki Kotone.
The following day, Kotone, who is revealed to be in the same class as Yuji, returns his umbrella. She is the younger sister of Fujisaki Ayane, a renowned beauty and the "idol" of their school. Yuji, still mortified by his previous day's actions, is further flustered by Ayane's presence and her brief interaction with him. Kotone explains that she knew Yuji's name because her sister, Ayane, mentioned him as being in the same class. This revelation surprises Yuji, who hadn't realized Ayane had a sister.
Later, Yuji finds himself with free time due to a friend's cancellation of plans. He visits a maid cafe, a novel experience for him. There, he is unexpectedly served by Kotone, who is working there in secret. Yuji realizes that the school prohibits part-time jobs, and Kotone is in a precarious situation if discovered. To protect her secret, Yuji proposes a deal: he will keep her secret if she agrees to be his girlfriend until the summer break. Kotone, despite her fear and the coercive nature of the proposal, accepts.
The next day, Yuji's friend, Ide, witnesses Kotone bringing Yuji a handmade bento, leading him to believe they are in a genuine romantic relationship and that Yuji coerced her. Ayane, overhearing Ide's confused mutterings and having seen Kotone prepare an extra bento, confronts Yuji. Yuji, to deflect suspicion, claims the bento was a thank-you gift for lending Kotone his umbrella. Ayane, though initially suspicious, accepts his explanation.
During lunch break, Kotone joins Yuji and Ide, revealing she made him a bento. Yuji, though pleased, is surprised by the simple, health-conscious contents of the bento. Kotone confesses it's her first time making a bento for someone, and Yuji explains the significance of a handmade bento to boys. As the lunch break nears its end, Kotone asks Yuji if they should keep their relationship a secret, and then directly asks if he likes her sister, Ayane. Yuji truthfully denies liking Ayane romantically, but he omits the fact that he had previously confessed to Ayane. The chapters conclude with Yuji and Kotone parting ways to return to class, their unusual relationship established under the shadow of Kotone's secret part-time job.