Ichiro Sato is an academically, athletically, and physically average third-year student at Kawanuma West High. His initial romantic interest in classmate Aya ends after he defends his Japanese language teacher, Kana Kojima, earning Aya's dislike. Ichiro repeatedly encounters Kojima in compromising situations – accidental meetings in places like the men's bathroom, laundromat, school infirmary, and pool, often involving unintended nudity or physical contact that escalate through comedic misunderstandings. Despite Kojima's fearsome "Kojima the Demon" reputation among students, Ichiro discovers her underlying shyness and clumsiness.

Their relationship deepens when Ichiro learns he unknowingly shaped Kojima's career path: as a child, he encountered her during a vulnerable moment when she lacked confidence and wore glasses. His encouragement motivated her to become a teacher, though he initially forgot this meeting. This revelation shifts their dynamic from awkward encounters to genuine romantic interest. Ichiro's family further connects their lives; his mother, Saki Sato, is Kojima's childhood friend, facilitating interactions outside school, and his younger sister, Shio, also appears in their personal lives.

After high school, Ichiro enters university via recommendation and moves into his own apartment. Kojima visits him frequently, their relationship progressing to a committed partnership. They eventually become engaged and live together, marking a significant evolution from their initial student-teacher dynamic. Throughout, Ichiro consistently demonstrates responsibility and kindness, aligning with Kojima's preference for a younger, dependable partner good with children. His actions consistently aim to protect and support Kojima as their circumstances transition from high school to adulthood.