The story begins in Kawagoe, a town known for its Western-style confectionery shops and its private academy. Haruo Hibiki, a former conductor who was dismissed from the Cosmopolitan Philharmonic Orchestra, receives a job offer from his grandmother, the principal of Kawagoe Private Academy. She suggests that he establish a boys' choir at the school to improve his communication skills and restore his professional reputation. If he succeeds, she promises to help him return to work as a conductor.

Haruo reluctantly accepts the challenge and begins recruiting students. First, he meets Tenshi, a young boy who loves to sing but is too shy to perform in public. Tenshi's mother reveals that he sings inside a soundproof cardboard box at home, which Haruo dubs "Cardbocchi." Haruo also encounters two other students—one from the swimming team and another who works as a fill-in for various clubs. Despite their initial reluctance, they agree to join Haruo's ambitious project.

The school's music teacher, Iijima Rika, joins the club as an advisor. She assigns the boys a song to learn, giving them two hours to prepare for a performance. Haruo emphasizes the importance of singing from the diaphragm and communicating with their entire bodies. After some awkward and hesitant attempts, the boys manage to deliver a harmonious performance.

Though Haruo is eccentric and hard to understand, he succeeds in conveying his passion for music. He explains that the world is made of musical notes and that their lives will brighten if they embrace the wonder of music. Despite his unorthodox teaching methods, the episode ends with the boys somewhat convinced, and the choir takes its first steps.

Thus, the stage is set for the Kawagoe Boys Sing—a choir composed of ordinary boys but led by a man with extraordinary ambitions.