"God Mars: Untold Legend" is a science fiction mecha anime that centers on the conflict between Earth and the planet Gishin, a technologically advanced civilization seeking to conquer humanity. The story begins with the birth of Takeru Myojin, a child genetically engineered by Gishin to serve as a weapon against Earth. However, Takeru is raised on Earth by a loving family, unaware of his origins. As he grows older, his latent abilities and connection to the powerful mecha God Mars begin to surface, revealing his true purpose.

The narrative unfolds as Gishin launches an invasion of Earth, deploying advanced mecha and soldiers to subjugate humanity. Takeru, now a teenager, is thrust into the conflict when he discovers his ability to pilot God Mars, a colossal robot created by Gishin but now used to defend Earth. Alongside his allies, including his childhood friend Marg and the Earth Defense Force, Takeru battles Gishin's forces while grappling with his identity and the moral implications of his existence. The series explores themes of loyalty, identity, and the struggle between destiny and free will as Takeru confronts his creators and fights to protect the planet he calls home.

Key characters include Takeru, whose internal conflict drives much of the narrative; Marg, who provides emotional support and serves as a grounding force; and the leaders of Gishin, who represent the cold, calculating logic of their civilization. The Earth Defense Force plays a crucial role in the war effort, showcasing humanity's resilience and ingenuity in the face of overwhelming odds. The series balances intense mecha battles with moments of introspection, delving into the psychological toll of war and the complexities of Takeru's dual heritage. The overarching plot builds toward a climactic confrontation between Takeru and Gishin, culminating in a resolution that addresses the themes of unity and the potential for redemption.