Yū Arimura, known professionally as Yu Otonashi, served as the center of the popular idol group "Mr. Doctor" before his sudden retirement from the entertainment industry. This career shift was sparked by witnessing a transformative horse race, inspiring him to pursue becoming a professional jockey. His decision triggered significant backlash from fans and industry figures, including covert sabotage attempts by his former manager aiming to derail his new path and pressure his return to idol work.

He enrolled as a first-year student at the prestigious JRA Horse Racing School, joining peers Shun Kazanami, Amane Grace, Aki Kyoriki, Hayato Hosho, Sojiro Sakuraba, Kota Maki, and Eri Shimotsuki. Transitioning from idol life demanded adaptation to the rigorous physical and academic challenges of jockey training. During this period, he sustained a permanent injury requiring lifelong prescription medication, a fact he acknowledged openly without shame. His development involved forging bonds with horses and peers, including forming a close relationship with Shun Kazanami, who possessed an innate ability to communicate with horses. Their dynamic evolved from initial rivalry into deep mutual reliance, culminating in a promise of unconditional support.

He confronted the controlling nature of the entertainment industry he left behind while navigating the intense physical and psychological pressures of jockey training. His journey emphasized perseverance amid external skepticism and internal growth.