"Oni-Tensei" is a supernatural action anime set in a world where humanity is under constant threat from demonic entities known as Oni. The story centers on Haruto, a young man who becomes the vessel for the reincarnated soul of a legendary Oni slayer, Raiden. After a catastrophic event unleashes a horde of Oni into the human world, Haruto discovers his latent connection to Raiden and is thrust into a battle to protect humanity. As Haruto learns to control the immense power within him, he is guided by a secretive organization dedicated to combating the Oni threat, led by the enigmatic strategist, Akira.

The narrative explores Haruto's struggle to balance his humanity with the growing influence of Raiden's spirit, which grants him extraordinary combat abilities but also risks consuming his identity. Alongside Haruto is Yuna, a skilled warrior with a tragic past tied to the Oni, and Kenta, a tech-savvy ally who provides tactical support. The trio faces increasingly powerful Oni, each with unique abilities and motivations, while uncovering the truth behind the Oni's resurgence and their connection to ancient human-oni conflicts.

The plot delves into themes of identity, sacrifice, and the blurred line between good and evil, as Haruto and his allies confront not only external threats but also internal conflicts. The story builds toward a climactic confrontation with the Oni overlord, Shuten, whose plans threaten to plunge the world into chaos. Through intense battles and emotional revelations, "Oni-Tensei" weaves a tale of heroism and the enduring struggle between humanity and the supernatural.