Miyoshi has spent his entire school life competing with his childhood friend Mamiya, only to lose every time. Mamiya excels at academics, sports, and everything else, and the rivalry defines their relationship. One day Mamiya collapses from an illness and is hospitalized for several weeks. When he returns to school, his appearance has changed: longer hair, a more delicate frame, and a distinctly feminine figure. It turns out Mamiya has undergone what is known in this world as emergence, a biological process in which a genetic predisposition causes some boys to transform into girls during adolescence. The change is effectively permanent, and society recognizes it as a natural, if unusual, phenomenon. Mamiya decides to try living as a girl, much to Miyoshi's horror.

The story takes place in a contemporary Japanese school setting, where this transformation is not entirely unheard of. The narrative follows Miyoshi as he struggles to reconcile his feelings toward the person who was once his rival and is now a girl. Despite his initial shock and his self-professed dislike of girls, Miyoshi gradually comes to terms with the new situation. The relationship between the two deepens, moving from rivalry to romance, and the series traces their journey through high school, university, and beyond. Notable arcs include their secret dating while in university, the disapproval of Miyoshi's cousin Takehiko, meeting each other's families, and eventually marriage and parenthood. The transformation is treated not as a temporary gag but as a lasting change that forces both characters to rethink identity, attraction, and what it means to love someone.

The main characters are Miyoshi, who is prickly and competitive but grows more caring, and Mamiya, who adapts to her new body with surprising ease and maintains her sharp intellect and confident personality. Other key figures include Takehiko, Miyoshi's protective older cousin, and later a new generation of characters in the spin-off series Kanojo ni Naru Hi Another, which focuses on a middle school boy named Sagara who emerges and must navigate his feelings for a female classmate while struggling to accept his own transformation. The original series spans twenty chapters collected in four volumes, covering the full arc from Mamiya's emergence to the couple's married life.