Kanzaki, a human student navigating ordinary life, finds his routine disrupted upon receiving an anonymous love letter from Mari Setagoya—a vampire-succubus hybrid reliant on "male vitality" for survival. Shy and socially unskilled, he struggles to process Mari’s abrupt, confrontational advances, which escalate into physical clashes. Reluctantly, he negotiates a transactional arrangement, offering a substitute bodily fluid to meet her needs while avoiding deeper entanglement.

External pressures destabilize their fragile dynamic, exposing Kanzaki’s insecurities as rivals challenge his role in Mari’s life. Plagued by doubts about his worth, he wavers between retreat and tentative connection, only for Mari’s unwavering persistence to counter his hesitations. Their interactions gradually erode his passivity, sparking flickers of assertiveness—he initiates physical closeness during vulnerable moments, testing boundaries of shared desire.

Caught between gnawing self-doubt and deepening emotional ties, Kanzaki confronts crises that jeopardize their bond. This tension crystallizes into a deliberate choice to embrace exclusivity, prioritizing their unconventional partnership despite its risks. His arc traces a jagged path from skittish avoidance to guarded agency, framed by a relationship that blends chaotic humor, reluctant vulnerability, and interdependence forged through clashing needs.