Sosuke Ajino, a rising star in Japan's classical music scene poised for international piano acclaim, owned a unique custom-made grand piano with an exceptionally heavy keyboard only he could play properly, yielding extraordinary sound. A severe car accident abruptly ended his promising career, injuring his left arm and killing his fiancée. This tragedy plunged him into despair, stripping away everything he cherished. Unable to perform yet incapable of abandoning music, he sold his prized piano to a nightclub in Morinohata's red-light district and began teaching at a music university.

Despite his position, Ajino struggled for a decade to find students whose musical interpretations he deemed compelling or interesting. Disillusioned with academia, he resigned and journeyed to locate his former piano. Tracing it to the Morinohata forest, he discovered it damaged and unplayable. Unable to produce sound from it yet unwilling to leave, he settled nearby as an emotionally detached elementary school music teacher, his engagement minimal until encountering a student, Kai Ichinose.

Ajino observed that Kai, a boy from the red-light district, could play the forest piano—an instrument others perceived as broken. Recognizing Kai's raw, instinctive talent and ability to perfectly replicate any piece after hearing it once, Ajino became a dedicated mentor. He convinced Kai to accept formal lessons by demonstrating Chopin's Minute Waltz, a piece Kai couldn't master without technical guidance. Ajino focused instruction on refining Kai's technique while fostering his confidence and emotional connection to music. Serving as a surrogate father figure, he prepared Kai for the International Chopin Piano Competition in Warsaw, channeling his own unrealized artistic aspirations through his protégé and helping Kai navigate personal challenges, including his impoverished background.

Key to Ajino's influence were pieces symbolizing their relationship; he performed "Little Brown Jug" during their first meaningful encounter, a tune Kai later associated with pivotal moments. Ajino's analytical approach and insistence on technical precision contrasted with Kai's intuitive style, yet he respected Kai's need to develop a personal musical identity. His past indirectly resurfaced when Pang Wei, a Chinese competitor at the Chopin Competition, displayed a piano style eerily reminiscent of Ajino's pre-accident playing. Ajino acknowledged the similarity but encouraged Pang to evolve beyond imitation. Through guiding Kai, Ajino reconciled his traumatic loss and career-ending injury, finding renewed purpose in nurturing the next generation of musicians. His journey reflects themes of artistic legacy, mentorship transcending personal tragedy, and music as a conduit for emotional healing and connection.