"Detonator Orgun" is a three-episode OVA series set in a near-future Earth, where humanity faces an existential threat from an advanced alien race known as the Evoluders. The story begins in 2026, with Earth under siege by these extraterrestrial invaders, who possess highly advanced technology and a relentless drive to conquer. The protagonist, Tomoru Shindo, is an ordinary college student who becomes entangled in the conflict after encountering a mysterious woman named Sonoi. She reveals that Tomoru is the chosen pilot for Orgun, a powerful biomechanical battle suit designed to combat the Evoluders.

Orgun, a sentient weapon of immense power, bonds with Tomoru, granting him extraordinary abilities but also burdening him with the responsibility of defending Earth. As Tomoru struggles to adapt to his new role, he learns that the Evoluders are not merely invaders but a race seeking to evolve by assimilating other species. Their ultimate goal is to absorb humanity's genetic material to further their own evolution. The series explores Tomoru's internal conflict as he grapples with the moral and emotional weight of his mission, balancing his humanity with the demands of being a warrior.

Supporting characters include Yoko Katsuragi, a skilled pilot and Tomoru's childhood friend, who provides emotional grounding and camaraderie. Sonoi, the enigmatic woman who introduces Tomoru to Orgun, serves as a guide and mentor, though her motives remain ambiguous. The Evoluders, led by their commander, Zell, are portrayed as a formidable and relentless force, driven by a cold, logical pursuit of survival and dominance.

The narrative unfolds through intense battles between Orgun and the Evoluders, showcasing the series' focus on mecha combat and strategic warfare. As the conflict escalates, Tomoru uncovers deeper truths about the Evoluders' origins and their connection to Orgun, leading to a climactic confrontation that tests his resolve and humanity. The series concludes with a resolution that underscores themes of sacrifice, evolution, and the enduring struggle to preserve one's identity in the face of overwhelming odds.