Two seemingly ordinary but keenly observant campus teenagers step into an ancient castle deep in the mountains because of a yellowed manuscript of a nursery rhyme. The manuscript leads to the mysterious death of a guest ten years ago. They must solve the secret of seven notes within three days, otherwise the person who listens to the entire lullaby will become the seventh corpse. In the process, they not only had to overcome the psychological fear and clue-finding limitations caused by the eerie environment of the castle, but also misunderstandings and suspicions about each other due to misleading clues. As time approaches, the danger intensifies, and the inexplicable humming of children on the mobile phone follows them. Although they succeeded in preventing the tragedy in the end, they also left indelible psychological trauma.
