Eun Dan-oh is a high school student from a wealthy family who suffers from a lifelong heart condition. One day, she discovers that she is actually a character in a webtoon titled 'Secret' and that her life is being dictated by the writer. Dissatisfied with her predetermined role as a supporting character who is destined to die young, she decides to forge her own destiny by changing the plot and finding her true love. As she navigates the boundaries between the 'stage' (where characters follow the writer's script) and the 'shadow' (where they have free will), she meets a nameless student, whom she names 'Ha-ru'. Together, they attempt to defy the writer's narrative, but their interference triggers unforeseen consequences that threaten the stability of their world and the lives of those around them.