Niki Yuichi is a human student at Myōjin University subjected to relentless bullying and social isolation. He develops an obsessive infatuation with classmate Akemi Itō, fueling intense jealousy toward her boyfriend Tatsuo Nagumo, whom he perceives as a rival. Severe abuse marks his home life, particularly from his alcoholic father who physically assaults him after discovering Niki stole money from his wallet.

Demons identify Niki's vulnerabilities and manipulate him into becoming an instrument against the Chōjin prophecy. They offer him power through a demonic phallus, demanding self-castration as the price for transformation. In a pivotal act of bodily mutilation, Niki severs his own genitalia to accept the demonic entity.

Following the transformation, Niki undergoes drastic physical changes, developing a hyper-masculinized, muscular physique and aggressive demeanor. He engages in acts of sexual violence and hedonism, including assaulting multiple women and orchestrating a group assault where females taunt him as "dickless wonder" during an extended lesbian encounter. Intermittent psychological conflict surfaces, notably when he apologizes to a victim during an assault after her resemblance to Akemi triggers momentary remorse.

Under demonic influence, Niki murders both of his parents, an escalation cementing his descent into inhumanity. His obsession culminates in kidnapping and attempting to rape Akemi at a construction site, during which he ingests Nagumo's bodily fluids to regenerate his demonic phallus. This confrontation leads to his demise when Nagumo, transformed into the Chōjin, kills him by tearing out his internal organs.

Niki's narrative functions as a tragic corruption arc, illustrating the destructive consequences of exploiting vulnerability for power. His physical and psychological disintegration serves as a foil to the apocalyptic events surrounding the Chōjin's emergence.