S1E615 : The Secret the Diary Plays (2)

Realizing the piano playing they hear is a recording, they enter the piano room. Conan notices the piano chair was set to a child's height and deduces where the boy is. He has the Detective Boys sing a song in the piano room and catches the boy watching them and befriends him. The boy, Keita Onoda, is a piano prodigy to which the kidnapper abducted to allow his daughter to win a piano competition. Conan also reveals the dairy entry is out of order due to Keita's tampering and that the kidnapper felt increasingly guilty as the days went on. After reading the last page in the dairy, they realize the kidnapper plans to suicide in the bathroom and manage to save him. In the aftermath, Keita's mother does not push charges due to Keita's defense. The kidnapper's daughter meanwhile, revealed she skipped the piano competition as she was worried about Keita's disappearance.