This animation about a feisty boy, Kai Ichinose, who finds an abandoned piano in a forest and learns to play it by himself, is so addictive I watched the whole 2 seasons, of 12 episodes each, in one sitting.
He rescues a classmate Shuhei Amamiya from bullies and, after finding out Shuhei wants to be a pianist, shows his new friend his piano in the forest. By some magic, the piano only responds to his touch. Shuhei is startled by Kai's playing and starts to harbor a complex emotion of envy and admiration for Kai, an emotion that eventually turns into an obsession to prove his superiority in piano playing.
Kai's teacher, Sosuke Ajino, a once-renowned pianist who got incapacitated after an accident, finds out about Kai's piano and volunteers to train the poor but extremely talented boy who, being the son of a prostitute and raised in a red-light district, would have little chance to better himself.
Season 2 is mostly about the International Chopin Piano Competition that Kai and Shuhei participate in. Side stories about the participants and Ajino add to the delightfulness of the series. The competition is fierce, raising my respect for pianists through the roof for having not only enormous talent but also incredible memory and nerves of steel. The commentaries from spectators add to the excitement about the events. But most of all, the music is exquisite, made more so by the pianists' affinity with it.