Optimistic, 16-year-old Nanako Misonoo anticipates a blissful experience at her new high school. Enrolling alongside her best friend Tomoko Arikura, she is confident that her freshman year will be exciting and filled with pleasant memories.

However, upon entering the prestigious all-girls Seiran Academy, the wondrous school life Nanako had envisioned quickly vanishes as she is inducted into one of the school's most elite clubs, the Sorority. Normally only accepting the wealthiest and most prominent students, Nanako—who is neither rich nor well-known—is rumored to have cheated the selection and becomes a target of bullying from her envious peers. To make matters worse, she gradually becomes involved with Seiran's most popular students, the "Magnificent Three," comprising of the disconcerted pianist Rei Asaka, the competitive tomboy Kaoru Orihara, and the dignified Sorority President Fukiko Ichinomiya.

As a result of her abnormal situation, Nanako begins documenting her daily life in the form of letters to her former teacher, Takehiko Henmi, whom she fondly refers to as "brother," all while striving to find peace amidst the turmoil that is Seiran Academy.

Included one-shot: Volume 3: Early Spring Trembling

Oniisama e... was published in Polish by Japonica Polonica Fantastica in May 2014; and in Spanish by Arechi Manga on October 10, 2024.