Preschool teacher Serena was tricked by her gambling-addicted father into an underground black market auction, where she was put up for public sale to pay off his debts. In her darkest moment of utter humiliation, Cassian, a mafia don, burst into the auction house to seek revenge. He killed his enemies, saved her, and took her for himself. Cassian made Serena drink a glass of red wine supposedly laced with a lethal neurotoxin, then locked a gemstone necklace that monitors her heart rate in real time around her neck: if she falls in love with him within thirty days, the poison will kill her; if she guards her heart, she can leave freely. During their forced cohabitation, Cassian gradually breaks down Serena's defenses with playing hard to get, overbearing protectiveness, and clumsy deep affection, while the necklace faithfully records every time her heart races, every flush on her cheeks, and every wave of longing that surges in the dead of night. When the enemies' final counterattack drives the two into a desperate corner, Serena shoots dead the foe threatening Cassian's life, and says through tears, "I love you, I don't care even if the poison kills me for it." Cassian tells her with a smile: there was never any poison in that glass of wine, but every one of her heartbeats was real.
