"Cosmo Warrior Zero Gaiden" is a two-episode OVA series that serves as a prequel to the "Cosmo Warrior Zero" anime, expanding on the backstory of its titular character, Captain Harlock's rival, Captain Zero. Set in a dystopian future where Earth's government has become oppressive and corrupt, the story explores Zero's transformation from a loyal soldier to a disillusioned rebel.

The narrative begins with Zero as a high-ranking officer in the Earth Defense Force, tasked with maintaining order across the galaxy. His unwavering loyalty to Earth is tested when he uncovers the government's involvement in unethical experiments and the exploitation of colonies. These revelations clash with his sense of justice, leading to a crisis of conscience. The turning point occurs when Zero is ordered to suppress a rebellion on a distant planet, only to discover that the rebels are fighting against the very corruption he has begun to despise. This mission forces him to confront the moral ambiguity of his role and the true nature of the government he serves.

Key characters include Zero, whose internal struggle drives the narrative, and his loyal crew, who grapple with their own loyalties as they witness his transformation. The series also introduces antagonists within the Earth government, whose actions highlight the systemic corruption that fuels the conflict. The OVA delves into themes of betrayal, redemption, and the cost of standing against tyranny, providing a deeper understanding of Zero's motivations and the events that shape his eventual rebellion.

The plot is tightly focused, with a strong emphasis on character development and moral dilemmas. It balances action sequences with introspective moments, offering a nuanced portrayal of Zero's journey from a dutiful soldier to a defiant leader. The series complements the broader "Captain Harlock" universe while standing on its own as a compelling exploration of its protagonist's origins.