Unknown to Everyone

Chae Yeon-ah goes to a blind date in place of her sick younger sister, Chae Sia. The date is with Seo Jeong-ju, the heir to the Sunghwa Group and Sia’s long-time crush. Jeong-ju falls for Yeon-ah and rushes the marriage, leading Yeon-ah to even go through with the wedding ceremony on her sister's behalf. Later, when a recovered Chae Sia moves into Jeong-ju’s home, he begins to feel a strange sense of dissonance in their married life. Amidst this, a slip of the tongue by their father, Chae Chung-soo, who mistakes Yeon-ah for Sia, reveals a shocking truth: Yeon-ah was intentionally abandoned at an orphanage and has been used solely as a 'kidney donor' by her father and sister. When Yeon-ah threatens to expose them, Chae Chung-soo, eyeing a presidential run, attempts to eliminate her. However, due to a hitman's mistake, Sia suffers an accident instead and falls into a coma. Yeon-ah then decides to live as Chae Sia and plot her revenge against her family.