After a ruthless hitwoman named Hae-Gu kills a man in front of his daughter and adopts the child, whom she names Mian (meaning 'sorry' in Korean), she sets a chilling task for the girl: to kill her when she's able. Mian, plotting her revenge, plans to use a facade of affection and psychological manipulation to fulfill her mission. However, as Mian matures, her scheme blurs into a tumult of desire, potentially evolving into genuine affection. Meanwhile, Hae-Gu navigates conflicting emotions towards Mian amidst perilous assignments. Will their twisted tale lead them to authentic love, or will they remain entangled in a destructive dance of vengeance and violence?