The story begins with the protagonist, Tenma Haruka, a high school student, participating in a grueling summer track and field practice. His only solace is the arrival of a beautiful transfer student, Kozakura Haruka, who excels at the high jump. While the other boys are fixated on Haruka's physique, Haruka is drawn to her graceful movements. After practice, Haruka is tasked with cleaning up the high jump equipment alone because he was the only one to fail the jump. Kozakura offers to help, explaining she plans to join the track and field club. While moving equipment, Kozakura trips and falls into Haruka, leading to an awkward, close physical encounter. Their predicament worsens when other students lock them inside the sports equipment shed, trapping them as evening falls.
As night sets in and the heat subsides, Haruka and Kozakura bond over shared interests like video games and anime. A moment of romantic tension arises, leading to a near-kiss, which is interrupted by Haruka's father and the school principal. To escape the principal's accusations of inappropriate behavior, Haruka impulsively declares that he and Kozakura are married. His father, understanding the legal implications of their age and the situation, supports the lie. The principal, though skeptical, agrees to drop the matter if they can prove their marriage. This forces Haruka and Kozakura into a desperate situation: they must actually get married to avoid expulsion.
The following day, Haruka and Kozakura, accompanied by Haruka's father and grandfather, head to the city hall to file their marriage registration. They are pursued by the principal, who remains suspicious. Haruka's grandmother intervenes, distracting the principal while Haruka and Kozakura complete the paperwork, officially becoming husband and wife. They then proceed to Kozakura's luxurious apartment, where Haruka's father insists he stay to avoid further suspicion. Kozakura, who lives alone and comes from a wealthy family of company presidents, agrees to let Haruka move in. They begin to navigate their new life together, facing challenges like Kozakura's disorganization and their own awkwardness as newlyweds.
The narrative continues with Haruka and Kozakura settling into their shared life. Haruka discovers Kozakura's parents are both presidents of companies, explaining her affluent lifestyle. They decide to adopt Haruka's surname, Tenma, for their marriage registration. Their first day together involves shopping for necessities at a large store, where they playfully decide who will be the primary breadwinner through a game. Haruka learns that Kozakura has moved frequently due to her parents' careers and has attended all-girls schools, explaining her limited experience with boys. Haruka also learns that a classmate, Shiina Aoi, has a crush on him, which initially causes Kozakura distress but is quickly resolved as Haruka reassures her of his feelings. The chapters conclude with them preparing for their new life together, with Haruka's father suggesting a honeymoon to deepen their relationship.
