Hayato Izumi holds the vice-presidency of Seifu High School's student council, having lost the presidential election to Ui Wakana and her "liberalizing love" platform involving condom distribution. His life is upended when Ui moves into his apartment due to an intoxicated childhood marriage pact arranged by their parents. This cohabitation must stay hidden from the school.

Orphaned of his mother and living alone while his father works abroad, Hayato cooks and cleans for himself. Initially resistant to the arranged marriage, he slowly forms genuine respect and romantic feelings for Ui, though her persistent, clumsy sexual advances leave him both aroused and flustered. His internal conflict surfaces physically through nosebleeds and symbolic "Bad Angel" visions during intimate moments with Ui or other girls.

Hayato's interactions with Rin Misumi, the disciplinary committee head opposing Ui's policies, spur further development. After repeated accidental compromising encounters—like taking shelter with Rin in a love hotel during rain—he fixates on Rin's large breasts. This attraction creates a love triangle, escalating when Rin permits him to touch her breasts at a summer festival, nearly leading to sex before he retreats to Ui. Ui becomes aware, fueling her jealousy and efforts to secure Hayato.

A revealed childhood memory shows Ui expressing her desire to marry Hayato, strengthening their bond. Despite temptations and challenges—including Ui's deceptively youthful parents and Rin discovering their cohabitation—Hayato commits to Ui. His arc concludes with a public marriage proposal at school, which she accepts. Throughout, he navigates accidental pervert scenarios, acts as the straight man to Ui's antics, and balances responsibilities amid romantic entanglements.