Hina is a high school girl left utterly alone after the death of her grandmother, who raised her following the loss of her parents. Overwhelmed by grief and the fear of being alone, she recalls her grandmother's wish to see her in a wedding dress before dying. On the night before the funeral, Hina sneaks into the church, dons a veil, and recites wedding vows toward her grandmother's coffin. At that moment, a mysterious man suddenly appears. He is Lucifer, a strict and serious demon from the demon world. Through a series of misunderstandings, the two end up forming a mistaken contract that binds them together, making Lucifer Hina's official guardian. Now Hina must navigate daily life with a no-nonsense demon acting as her protector and housemate, all while dealing with the growing complications of their unusual arrangement. The story unfolds as a romantic comedy, exploring the awkward cohabitation between a sheltered human girl and a straight-laced demon who has no idea how to handle human emotions. As their relationship deepens, interference from Hina's fiercely protective best friend Hagane and the arrival of other demons, such as the scheming Mephisto, create further conflict and drama. The narrative arcs follow the pair as they gradually move from a transactional contract to genuine care, with moments of comedy, supernatural trouble, and romantic tension against the backdrop of both the human world and the demon realm.