Mu Shengsheng is the frail daughter of the Marquis of Xuanwu, born with a terminal illness that doctors predict will kill her before her sixteenth birthday. She possesses a mysterious ability to see a countdown of her remaining days floating above her head, a number that decreases by one each day. As her sixteenth birthday approaches, her lifespan dwindles to a mere three days. Her stepmother, Lin, who controls the household in the Marquis's absence, devises a cruel plan to profit from Mu Shengsheng's impending death. She arranges a ghost marriage between the dying girl and the deceased Second Master of the Duke of Zhenguo, a notorious and dissolute man who died of disease. The plan is to marry Mu Shengsheng to the corpse and bury her alive alongside him, allowing Lin to claim a substantial dowry from the powerful Duke's family.
On the day of the forced wedding, Mu Shengsheng is dressed in bridal attire and placed in a sedan chair. Realizing the horror of her fate, she summons her remaining strength to escape, rolling out of the moving carriage into the busy streets of the capital. As she flees, a strange voice guides her, urging her to run. In her panic, she collides with a man, an encounter that unexpectedly increases her remaining lifespan from three days to four. This man is Lan Xun, the Regency Prince, the most powerful and feared figure in the empire, known as the Living Yama King. Mu Shengsheng realizes that physical contact with Lan Xun extends her life, as a surge of energy flows from him to her upon touch.
Desperate for survival, Mu Shengsheng clings to Lan Xun, begging for protection. Lan Xun, initially cold and dismissive, agrees to let her stay in his mansion for three days, partly out of disdain for the Duke of Zhenguo's household and partly because he finds her unusual. He moves her to a nearby courtyard to keep her close while maintaining distance. Mu Shengsheng uses every opportunity to get close to him, feigning dizziness and weakness to fall into his arms, thereby extending her life. Her actions initially irritate Lan Xun, who is a man of ascetic habits and has never taken a concubine. However, he finds himself increasingly affected by her presence, suffering from insomnia and vivid dreams after she delivers him calming tea.
The conflict escalates when the Duke of Zhenguo's household, led by Madam Cui, demands Mu Shengsheng's body for the ghost marriage. They argue that the marriage contract is valid and that Mu Shengsheng belongs to their family regardless of her survival. Lan Xun refuses to hand her over, stating that he will only deliver her body after her natural death. Meanwhile, Mu Shengsheng's condition worsens, and she collapses, vomiting blood. Lan Xun, concerned despite his earlier indifference, summons the legendary Divine Doctor Luo to treat her. Doctor Luo discovers that Mu Shengsheng's life has been extended significantly and identifies a rare ancient text, the Qing Nang Jing, as the source of a life-saving pill formula. Mu Shengsheng reveals that she had copied the text, which allows the doctor to begin crafting a cure.
As Mu Shengsheng recovers, her relationship with Lan Xun deepens. She confesses her love, claiming it was love at first sight, and Lan Xun, though skeptical, allows her to stay. The true extent of her dependence on him becomes clear when she kisses him while he sleeps, gaining several more days of life. The story sets the stage for further complications, as Mu Shengsheng's father remains missing, and various factions, including the Empress Dowager and the Duke of Zhenguo, plot against both the Regency Prince and the fragile girl who has become entangled in his world.
