Okuda is a single man in his thirties who works at a community-focused supermarket. Using a dating app, he miraculously connects with a young woman named Amateru Setsuna and the two begin dating. The central conflict of Rōninsei no Kanojo to Super de ignites when a new part-time employee joins Okuda’s supermarket. This new hire is revealed to be his girlfriend, Amateru, who is an eighteen-year-old rōninsei, a student not accepted to her first-choice university and spending additional time studying for entrance exams.

The series explores the complications of a secret workplace romance between two people with an age gap of nearly double. While Okuda and Amateru navigate the risk of their relationship being discovered, additional tension emerges from a female coworker named Izumi. Unaware that Okuda is already in a relationship, Izumi finds herself becoming increasingly conscious of him, setting up a potential love triangle.

The story unfolds across three volumes. Early in the relationship, Amateru asks Okuda what specifically he likes about her, and the answer he provides is not the correct one in her eyes. As time passes, the connection between Okuda and Izumi deepens unintentionally. After drinking together one evening, Izumi sees through Okuda’s feelings for Amateru. The narrative builds toward the return of university exam season, forcing both Okuda and Amateru to confront the future of their relationship as her long period of exam preparation potentially comes to an end. The story concludes with a focus on the year-end exam period and the ultimate fate of this secret workplace romance between two individuals whose ages are doubled apart.