In a world where vampires thrive on the life force of unsuspecting individuals, high schooler Karin Maaka stands out among her kind due to her rare condition of polycythemia. Afflicted with a disorder that causes her to produce excessive blood, Karin resorts to transferring her surplus blood to strangers to alleviate her symptoms, inadvertently bringing them joy and liveliness. Supported by her siblings Anju and Ren, who help conceal her actions, Karin's unique secret faces a threat when observant classmate Kenta Usui transfers to her school. As Karin's blood inexplicably reacts to Kenta, a complex and unforeseen connection unfolds, challenging the delicate balance of her hidden identity.