High school students Saito Hojo and Akane Sakuramori share nothing but mutual hatred. Known throughout their school as bitter rivals, they clash daily over test scores and trivial arguments. Saito is a dispassionate prodigy who neglects his own well-being, while Akane is a studious perfectionist with a fiery temper. Their hostile equilibrium shatters when Saitos grandfather, the head of a powerful corporate empire, delivers an ultimatum: marry Akane or be disinherited, with the family business left to a stray dog. To Saitos astonishment, Akane accepts without resistance.

The grandparents orchestrate the arrangement based on their own youthful romance. Saitos grandfather Tenryu and Akanes grandmother Chiyo were once lovers who could not marry due to family obligations. Now widowed and reunited, they force their grandchildren to live out the path they themselves could not take, pressuring the couple into immediate cohabitation in a newly built house. Tenryu threatens to disinherit Saito unless he complies, while Chiyo offers to pay Akanes expensive medical school tuition as incentive. The marriage documents are filed the same day, leaving the unwilling newlyweds trapped together.

Forced to share a home and even a bed, Saito and Akane must navigate domestic life while hiding their marriage from classmates. Their personalities clash constantly. Akane is a neat freak who demands order, while Saito leaves dishes piling up and ignores basic chores. Early interactions are filled with shouting matches and awkwardness, including the uncomfortable reality of sharing sleeping quarters as virtual strangers. Despite their animosity, both agree to keep their relationship secret, fearing the social fallout if their classmates discover they are married.

The narrative reveals that Akanes hostility stems from a forgotten childhood encounter. She and Saito met at a party years earlier and became fast friends, but Saito, despite his photographic memory, failed to recognize her later. Her anger masks deep hurt at being forgotten, and she has harbored feelings for him ever since. Saito, for his part, endured an abusive childhood with neglectful parents who treated him as a burden, leaving him emotionally guarded and accustomed to isolation. Their forced proximity slowly chips away at these defenses.

Several supporting characters complicate their situation. Shisei, Saitos younger cousin, develops an obsessive attachment to him, frequently sitting on his lap, calling him big brother, and even watching the couple from afar with a key to their house provided by the grandfather. Maho, another relative, appears later and attempts to seduce Saito, creating romantic rival tension. Akanes friend Himari develops a crush on Saito, unaware of the marriage, leading to a fake dating arrangement that stokes Akanes jealousy. Saitos abusive parents resurface later in the series, attempting to exploit the family situation for money.

Over the course of ten volumes, the relationship evolves from open hostility to grudging tolerance and eventually genuine affection. Key milestones include Akane falling sick and Saito carrying her to the hospital, Saito working to buy a wedding ring with his own money, and the couple defending each other against external threats. Akane experiences visible jealousy when other women show interest in Saito, while Saito becomes increasingly protective of her. The grandparents manipulative experiment begins yielding the results they intended, as two people who claimed to hate each other discover the love beneath their rivalry. The series concludes with Saito officially named successor to the family business, his parents permanently removed from the picture, and the unlikely marriage transformed into a genuine partnership.