The Chronological Truth: As a seven-year-old child, Sook-hee hides under a bed and hears her father murdered by a man whistling a distinct tune over a stolen jewel. Her father’s former associate, Jang-chun, kidnaps her and sells her to child traffickers. She is rescued by criminal assassin Lee Joong-sang, who raises and trains her; they eventually marry. During their honeymoon, Joong-sang is seemingly murdered by a rival gang, prompting Sook-hee's opening massacre in the drug den.
The Agency and Revelations: Recruited by Chief Kwon while pregnant, Sook-hee receives plastic surgery and becomes sleeper agent "Chae Yeon." She marries neighbor Jung Hyun-soo, unaware he was assigned by the agency to surveil her, though he falls in love with her genuinely. Later, Sook-hee discovers her new assassination target is Joong-sang, who faked his death. Horrifyingly, she learns Joong-sang was the whistler who murdered her father. Joong-sang's faction subsequently murders both her daughter Eun-hye and husband Hyun-soo.
Ending & Hidden Meaning: Sook-hee storms Joong-sang’s headquarters, pursues him through the city, and slits his throat with an axe on a crashed bus. As police surround her, she drops her weapon, looks directly into the camera lens, and lets out an unsettling, manic grin. This confirms Sook-hee has transcended all illusions of love, domesticity, and institutional control, fully claiming her moniker as "The Villainess" against the world that engineered her destruction.