Modern-day overworked office worker You Li wakes up to find herself transported into the body of an ancient assassin. With no cheat abilities to rely on, her very first mission goes horribly wrong when she accidentally attacks her notoriously ruthless master, the living King of Hell. To save her own life, she has no choice but to disguise herself as a cold, emotionless killing machine. While carefully carrying out assassination missions and navigating the competing forces around her, she secretly plots her escape by faking her death. Her scheming master relentlessly closes in, determined to keep her imprisoned at his side. Meanwhile, the Crown Prince, the young Marquis, the young lord of the Miao frontier, and others all fall for her. But You Li only wants to stay alive. She puts everyone who shows her affection on her danger list and remains firm in her resolve: an assassin doesn't fall in love.