Kousuke Kawakami is a generally kind-natured high school student tormented by recurring nightmares where he transforms into a demon and sexually assaults women, causing him deep psychological distress. The origin of these nightmares remains unexplained. His life takes a drastic turn when goddesses Fauna and Jordh confront him, revealing he unknowingly carries an embedded "devil's seed" or "seed of evil" threatening to awaken and potentially cause worldwide destruction. Witnessing an act of kindness—Kousuke helping Fauna down from a tree—stays their hand from immediate elimination. Instead, they move into his residence, accompanied by their younger sister, to monitor him closely and attempt to purify or suppress the demonic force within him. This purification process necessitates frequent sexual activity with the goddesses, presented as essential to prevent the seed's awakening.

Complicating his situation, an underworld emissary named Natsuki operates in disguise as his landlady. Her objective is to ensure the devil's seed awakens as planned, placing her in direct opposition to the goddesses' efforts. Kousuke's background as an ordinary student, previously unaware of supernatural forces, underscores his vulnerability. The narrative focuses on his internal struggle between his inherent humanity and the latent evil within him, mediated by the opposing actions of the goddesses and the underworld agent. It does not expand on his family history, personal interests beyond his immediate situation, or explicit character development.