My Demon follows the story of Do Do-hee, a cold-hearted and cynical heiress to a powerful conglomerate who trusts no one, and Jung Gu-won, a centuries-old demon who has lived by making dangerous deals with humans to sustain his existence. Their lives become inextricably linked when a mysterious incident causes Gu-won to lose his supernatural powers, which are transferred to Do-hee's wrist in the form of a cross-shaped tattoo. Forced into a contract marriage to protect their respective interests—Do-hee needs a bodyguard to navigate the treacherous power struggles within her family, and Gu-won needs to stay close to Do-hee to regain his lost abilities—the two embark on a tumultuous journey. As they navigate corporate conspiracies and supernatural threats, their initial animosity slowly transforms into a deep, fated romance that challenges the boundaries between the human and demonic worlds.