Mask follows the life of Byun Ji-sook, a woman struggling with her family's crushing debt who finds herself living the life of a wealthy heiress named Seo Eun-ha, who looks exactly like her. After Eun-ha dies under mysterious circumstances, Ji-sook assumes her identity to escape her past, eventually marrying Choi Min-woo, the heir to a powerful conglomerate who suffers from deep emotional trauma and paranoia. As Ji-sook navigates the treacherous waters of high-society politics and corporate power struggles, she must maintain her facade while dealing with Min-woo's suspicious nature and the manipulative schemes of Min Seok-hoon, the man who orchestrated the identity swap. The series explores themes of greed, identity, and the redemptive power of love as the protagonists struggle to reveal their true selves in a world built on lies.