"Only if you vow to keep this secret and continue serving me your blood as "food" for eternity..." Ruri, a weak and overworked office worker, finds solace in Gilbert, her favorite character from the otome game "Vampire Mansion." One morning, after pushing herself too hard, her co-workers find her collapsed at the office, and her body is already cold... However, when she awakens, she finds herself in the game world, and even meets Gilbert...! She has been reborn as Fiona, the fiancee who is destined to be killed by him! Fiona's struggle to avoid this fate and to be close to her beloved character begins. Over time, her feelings shift to wanting to save this man who has never been rewarded. Thus begins the story of the cursed vampire duke and the reborn side character.