Yingyu (Kim Ok-bin) who has a dual personality, was practicing vocal music at school late at night when she was slit in the throat by a flying sheet of music. Her voice echoed in the school, and only her good friend Shanmin (Seo Ji-hye) could hear it. Yingyu, who died unjustly, needs Shanmin's help to clarify the doubts about her death in order to be saved. Otherwise, five days later, when she is hoarse, her restless ghost will wander the school corridor forever until her soul is scattered. The school's music teacher (Kim Si-rong) was deprived of her singing ability by the disease. She could not face this cruel fact, so she looked for someone who could sing for her among her students. Yingyu was her favorite student. When all the evidence pointed to the music teacher, they discovered more unknown horror events hidden in the school.