Secret follows the life of Kang Yoo-jung, a kind-hearted woman who takes the blame for a hit-and-run accident committed by her boyfriend, a prosecutor. She ends up serving time in prison for a crime she did not commit, only to be betrayed by the man she sacrificed everything for. Upon her release, she finds herself entangled with Jo Min-hyuk, a wealthy and cold-hearted chaebol who lost his lover in that very same accident. Driven by a thirst for vengeance, Min-hyuk initially seeks to destroy Yoo-jung's life. However, as he digs deeper into the truth behind the accident, he begins to uncover the layers of deception surrounding her imprisonment. The series evolves into a complex melodrama where the lines between hatred and love blur, forcing both characters to confront their pasts and the people who orchestrated their suffering.