Atsushi Arisawa enters Jougaoka High fueled by childhood affection for his neighbor "Ma-Chan," believing her to be a teacher there. He discovers his crush, Masami Shibata, no longer teaches there, and instead, Masami's younger brother, Masayoshi Shibata, serves as the school nurse. Atsushi initially mistakes the similarly featured Masayoshi for Masami but becomes confused by Masayoshi's unexpectedly harsh and unrefined personality, starkly contrasting his cherished memories of the kind "Ma-Chan."

Born in Tokyo, Atsushi lives with his parents and a pet dog. Academically strong yet sometimes naive and lacking common sense, he is sympathetic and emotionally expressive, occasionally displaying a crybaby demeanor. Despite his initial confusion and disappointment over Masayoshi, Atsushi gradually develops genuine romantic feelings for him, shifting his affection away from Masami. He resolves to mature into an adult worthy of Masayoshi's love.

His high school life is complicated by Kouji Inagaki, a classmate and childhood friend. As children, Atsushi and Inagaki shared their first kiss and promised to marry. Inagaki remains intensely devoted, pursuing Atsushi persistently throughout high school, causing Atsushi discomfort. While Atsushi does not reciprocate Inagaki's romantic feelings, their complex history and Inagaki's advances create ongoing tension. Atsushi finds support from Yoriko Hayase, their classmate and self-proclaimed yaoi fangirl, who actively encourages both Inagaki's pursuit and later, Atsushi's relationship with Masayoshi.

From age sixteen to his early twenties, Atsushi undergoes substantial maturation, evolving from an emotionally volatile teenager into a more serious, understanding, and confident young adult. This growth is central to his relationship with Masayoshi. Initially perceived as the more submissive partner ("uke") in high school, their dynamic shifts as Atsushi ages. By adulthood, he adopts a more dominant role ("seme"), reflecting his increased assertiveness and emotional stability. He becomes a student teacher, interning at the all-girls Tokiwa School, where he ironically finds himself the object of student crushes, mirroring his own past experiences.

Atsushi's mother is depicted as kind and welcoming, often making treats for others, contrasting with Inagaki's harsh mother. His familial environment appears supportive. The story follows his journey navigating unrequited childhood affection, evolving romantic relationships, achieving personal independence, and solidifying his identity within his social circle.