Kei, a teenage protagonist, joins the organization Spirits primarily to escape academic pressures, becoming a pilot for the biomechanical weapon Devadasy. He relocates to a specialized facility housing multiple female candidates, including his childhood friend Naoki Matsudo. A psychic link connects Kei to Amala Minakushi, a blue-haired individual discovered alongside Devadasy in Tibet by Kei's grandfather twenty years prior. This connection manifests through recurring erotic dreams featuring Amala before their physical encounter.

Exposure to Spirits' specialized drugs and Devadasy's internal nanomachines alters Kei's behavior, intensifying latent aggression and sexual urges. These changes trigger a near-rape incident targeting Naoki. Following this, Spirits officer Misako Takashina engages sexually with Kei to obtain a sperm sample; analysis confirms nanomachine contamination. Despite his violence toward her, Naoki retains protective feelings for Kei. Kei displays tactical aptitude in combat, deducing an alien entity's weakened state, though psychological trauma from his own actions frequently disrupts his focus during battles.

Kei's complex relationship with Amala is heavily implied to be sexual, serving as a power source for Devadasy which harnesses the pilots' sexual energy. Ancient prophecies designate Kei and Amala for a predestined role in creating new life through Devadasy. This culminates in Kei becoming pregnant after a convergence involving Amala, Devadasy, and an alien entity that assaulted him. The pregnancy concludes with Kei giving birth to a biomechanical entity while piloting Devadasy in the finale. His personality is characterized by pervasive unpleasantness, encompassing flippancy, overt perversion, and manipulative treatment of others, especially Naoki; these traits exist before but intensify after exposure to Spirits' environment and systems.

The narrative concludes ambiguously after an alien mecha's destruction and Naoki's rescue. A final scene, potentially dreamt, depicts a younger Kei and Amala in a school setting where she laughs at him being disciplined.