"Tenchi Muyo! War on Geminar" is a 13-episode OVA series set in the "Tenchi Muyo!" universe, though it functions as a standalone story. The narrative centers on Kenshi Masaki, a 15-year-old boy who is transported to the fantastical world of Geminar, a land where advanced biomechanical weapons known as Sacred Mechanoids are used in warfare. Kenshi, the half-brother of Tenchi Masaki, is initially mistaken for an assassin sent to kill Princess Lashara Aasu of the Shtrayu Empire. After foiling the assassination attempt, Kenshi is taken into Lashara's custody and becomes embroiled in the political and military conflicts of Geminar.

Geminar is a world divided into multiple nations, each vying for power and control over the Sacred Mechanoids, which are piloted by individuals with special abilities. Kenshi, despite his young age, possesses extraordinary combat skills and a deep understanding of machinery, making him a valuable asset. He enrolls in Lashara's academy, where he trains alongside other students to become a Sacred Mechanoid pilot. However, Kenshi's pacifist nature and desire to avoid violence often clash with the expectations of those around him, creating tension as he navigates his new life.

The story explores Kenshi's relationships with the diverse cast of characters, including Lashara, who is determined to protect her kingdom; Wahanly Shume, a skilled pilot and Kenshi's rival; and Chiaia Flan, a noblewoman with a strong sense of duty. As Kenshi becomes more involved in the affairs of Geminar, he uncovers a larger conspiracy involving the origins of the Sacred Mechanoids and the true intentions of various factions. The series balances action, political intrigue, and lighthearted moments, with Kenshi's journey serving as the focal point.

The plot progresses through a mix of episodic adventures and overarching storylines, culminating in a climactic battle that tests Kenshi's resolve and abilities. Throughout the series, themes of loyalty, friendship, and the consequences of war are explored, with Kenshi's unique perspective as an outsider providing a lens through which the world of Geminar is examined. The series concludes with a resolution to the immediate conflicts, though it leaves room for further exploration of the world and its characters.