"Green Legend Ran" is a three-episode OVA series set in a post-apocalyptic world where Earth's environment has been devastated by an alien race known as the Rodo. The story unfolds in a barren landscape where water is scarce, and humanity struggles to survive under the oppressive rule of the Rodo and their human collaborators, the Holy Ra. The Rodo, who arrived during a catastrophic event called the Hammer of Eden, are worshipped as divine beings by the Holy Ra, who enforce their rule through religious dogma and military force.

The protagonist, Ran, is a young boy living in a small, impoverished village. His life changes when the village is attacked by the Holy Ra, and he encounters a mysterious woman named Aira, who possesses a pendant with the power to control water—a rare and valuable resource. Aira is revealed to be a member of the Hazzard, a resistance group fighting against the Holy Ra and the Rodo. Ran joins Aira and the Hazzard, driven by a desire to avenge his village and uncover the truth about the Rodo and their connection to Earth's destruction.

As the story progresses, Ran learns more about the Rodo's origins and their true intentions. The Rodo are revealed to be humans who underwent genetic experimentation to survive in the harsh environment of their dying home planet. Their arrival on Earth was not an act of conquest but a desperate attempt to find a new home. This revelation complicates Ran's perception of the conflict, as he grapples with the moral ambiguity of fighting against beings who are, in essence, refugees.

The series explores themes of survival, environmental collapse, and the consequences of humanity's actions. Ran's journey is one of self-discovery, as he confronts his own prejudices and learns to see the world beyond black-and-white morality. Aira serves as a mentor figure, guiding Ran while also dealing with her own past and the burden of her role in the resistance. The dynamic between Ran and Aira is central to the narrative, highlighting themes of trust, sacrifice, and the cost of rebellion.

The plot builds toward a climactic confrontation between the Hazzard and the Holy Ra, with Ran and Aira at the center of the conflict. The resolution of the series leaves some questions unanswered, emphasizing the ongoing struggle for survival in a world where hope is scarce but not entirely extinguished. The story's focus on environmental degradation and the cyclical nature of conflict provides a thought-provoking backdrop to the personal journeys of its characters.