"3x3 Eyes Seima Densetsu" is a supernatural action-adventure series centered around the journey of Yakumo Fujii, a young man living in Tokyo, and Pai, a mysterious girl who is the last surviving member of the Sanjiyan, a race of immortal three-eyed beings. The story begins when Yakumo is drawn into Pai's quest to regain her humanity, as she seeks to transform from an immortal Sanjiyan into a mortal human. This transformation requires the use of a powerful artifact known as the "Seima," which is scattered across the world in fragments. Yakumo, initially an ordinary human, becomes entangled in Pai's mission after a tragic encounter with a demon leaves him cursed with immortality, binding his life to hers.

As the narrative unfolds, Yakumo and Pai travel across various locations, encountering both allies and enemies. Their journey is fraught with danger, as they face off against demons, other Sanjiyan, and powerful supernatural entities who seek the Seima for their own purposes. The series explores themes of immortality, humanity, and the sacrifices required to achieve one's desires. Pai's dual nature as both a compassionate girl and a fearsome Sanjiyan creates tension, as her immense power often comes at a cost to those around her. Yakumo, on the other hand, struggles with his newfound immortality and the burden of protecting Pai while grappling with his own humanity.

The relationship between Yakumo and Pai forms the emotional core of the story, evolving from one of reluctant partnership to deep mutual reliance. Supporting characters, such as the demon hunter Benares and the enigmatic Kaiyanwang, add complexity to the narrative, each with their own motivations and connections to the Sanjiyan race. The series blends action, mythology, and emotional depth, creating a rich tapestry of supernatural intrigue. The quest for the Seima drives the plot forward, culminating in a climactic confrontation that tests the limits of loyalty, love, and the desire for redemption. Through its exploration of immortality and the human condition, "3x3 Eyes Seima Densetsu" presents a compelling tale of adventure and self-discovery.