Fifteen-year-old Rin Suzumiya enters the high school she has always dreamed of attending, only to find herself in the same class as Yukichi Sakamoto, the overbearing boy who picked on her relentlessly when they were in elementary school. Three years apart have changed nothing about his arrogant, domineering attitude. Desperate to get away from him, Rin absentmindedly scrawls her name on a piece of paper he pushes toward her, thinking it will finally make him leave her alone. She soon discovers that the document is a marriage registration form, and Yukichi flatly refuses to return it unless she agrees to stay by his side for three years. Trapped by her own careless signature, Rin becomes Yukichi's de facto personal assistant, forced to spend time with the person she most wants to avoid.

The story follows the push-and-pull between the kind-hearted, slightly naive Rin and the brash, possessive Yukichi, who has actually harbored a crush on her since childhood but expresses his feelings through teasing and forceful gestures. As Rin tries to devise a way to retrieve the marriage certificate, she begins to notice unexpected sides of him, while Yukichi, for all his bluster, sincerely wants her to choose to stay with him of her own free will. Their dynamic is further complicated by the arrival of Sawada, a mysterious and gentle male classmate who takes a clear interest in Rin, creating a love triangle that tests Yukichi's patience and brings his jealousy to the surface.

Across the series, Rin and Yukichi navigate summer festivals, school trips, cultural festivals, and overnight stays, each event drawing them closer even as misunderstandings and pride keep them at odds. Yukichi struggles with family pressure to enter an arranged marriage, while Rin wrestles with her growing awareness that her feelings for him are no longer purely hostile. After a series of near-confessions and emotional confrontations, the two finally become a couple. The story concludes with them as high school sweethearts, cherishing their first Christmas together and looking ahead to a future that fulfills the promise of the marriage certificate that started everything.