Shintarou Taki enters high school carrying a deep resentment toward music, largely because of his older brother Kotaro, a world-class saxophonist who has overshadowed him since childhood. Despite possessing absolute pitch and an almost supernatural ability to bond with any instrument, Shintarou has deliberately distanced himself from music and instead occupies himself with meticulous hobbies like making dioramas. While wandering the school festival in search of a club to join, his friend Yamada Kyousuke drags him to the wind instrument band—the Tanba Daisan High School brass band, affectionately called Tansansuibu. There, Shintarou discovers an old, neglected alto saxophone covered in patina and mold. When he meticulously cleans and polishes it, the instrument reveals itself as a vintage Selmer Mark VI—and, inexplicably, only Shintarou can see it as a young girl capable of speech. This mysterious personification of the saxophone, whom he nicknames Mark Six, becomes his partner as he reluctantly joins the band.

The brass band is a collection of eccentrics: the sleepy double‑bass player Koga Masa, who serves as club president; the stoic horn player who never speaks; the former baseball star turned trumpeter; the voluptuous tuba player; the twin clarinetists who practice rhythm games; the beautiful but enigmatic flute vice‑president Noguchi; and many others. Each member harbors their own quirks and talents, and the band carries a hidden secret that outsiders are not meant to know. As Shintarou gradually rediscovers his love for music through practice sessions, performances at the sports festival, and summer competitions, he must also confront the source of his hatred: his brother Kotaro, who now works as concertmaster of a professional wind orchestra and frequently visits the school to teach. The sibling rivalry escalates into open conflict, spilling over onto the band and even affecting Mark Six herself.

The narrative arcs follow Shintarou’s growth from a withdrawn, expressionless boy into a passionate saxophonist, the deepening bond between him and the personified saxophone, and the tangled relationships among band members. A love triangle forms between Shintarou and the Miyagi sisters—Michi, a kind‑hearted player from a rival school, and her younger sister—leading to a musical showdown at a summer festival. The final arc centers on the explosive confrontation between the Taki brothers, culminating in a climactic performance where Shintarou must decide whether to reject his past or embrace his talent and the instrument that has become his friend. Throughout the five‑volume story, the band’s daily absurdities are balanced with sincere dedication to music, and the secret behind the school’s legendary brass band is gradually revealed.