Forest of Piano is a story that follows Kai Ichinose, a boy who lives in the red light district but escapes at night to play the piano in the forest. Shuhei Amamiya, the grade-school son of a professional pianist, transfers to Moriwaki Elementary, Kai's elementary school. But it doesn't take long before Shuhei is picked on by the class bullies, and gets involved in a dare to play the mysterious piano in the forest, leading to his meeting with Kai, who seems to be the only one capable of getting sound out of the thought-to-be broken piano. Kai's ability earns him the respect of Shuhei and his music teacher, former master pianist Sosuke Ajino. Both Shuhei and Ajino try to get Kai to take proper piano lessons, but Kai is at first resistant to refining his piano-playing technique. However, after hearing Sosuke play a Chopin piece he just can't seem to play himself, he relents.
A piano prodigy who lost his ability to play after suffering a traumatic event in his childhood is forced back into the spotlight by an eccentric girl with a secret of her own.