Ella Black, a 28-year-old hematology research assistant, is crushed by the exorbitant medical costs of treating her ten-year-old sister Lily’s rare, intractable blood disorder. To save Lily, Ella enters into a one-year contract marriage with Adrian Thorne, a mysterious and reclusive billionaire. The agreement stipulates that she must donate 100 milliliters of blood weekly to the Thorne family blood bank; in exchange, Adrian covers the full cost of Lily’s transplant surgery and all medical treatments.
Upon moving into Adrian’s gloomy Gothic estate, Ella uncovers shocking secrets: Adrian never ventures out before sunset, every room is draped in blackout curtains, and the manor houses a vast stockpile of human blood. She soon learns that Adrian is a 327-year-old pureblood vampire lord afflicted by an ancient curse that causes his bloodline to steadily decay, eventually transforming him into a mindless, bloodthirsty monster.
Ella’s blood bears the legendary "Mark of Holy Blood," making it both the only natural antidote to the vampire curse and the key to curing Lily’s hereditary blood disorder. Initially, Adrian marries her solely for her unique blood, and Ella stays only for her sister’s sake. However, as they join forces to confront radical vampire aristocrats, the Vampire Council of Elders' draconian three-hundred-year-old laws, and the ghosts of Adrian’s tragic past, genuine love begins to blossom between them.
Leveraging her expertise in medical research, Ella analyzes the Holy Blood gene and develops a synthetic serum, eliminating the need to imprison humans for blood and breaking a centuries-old cycle of sacrifice that harmed both species. After surviving countless life-or-death perils, Ella tears up the short-term blood donation contract, choosing instead to spend eternity with Adrian. Together, they rewrite the destiny of the entire vampire race, and Lily is completely cured thanks to the Holy Blood gene.