"Karas" is a six-episode OVA series set in a fictional version of Tokyo, where humans coexist with supernatural beings known as Mikura, who are remnants of ancient spirits. The city is protected by a chosen guardian called the Karas, a powerful entity tasked with maintaining the balance between the human and supernatural worlds. The story begins with the betrayal of the current Karas, Eko, who abandons his duty and seeks to overthrow the human world, using his powers to create a mechanized army of Mikura. This rebellion disrupts the fragile equilibrium, plunging the city into chaos.

The narrative shifts focus to Otoha, a former human resurrected as the new Karas to confront Eko and restore order. Otoha struggles with his dual identity, torn between his human past and his new role as a supernatural enforcer. As the conflict escalates, Otoha uncovers the tragic history behind Eko's descent into darkness, revealing themes of loss, vengeance, and the consequences of abandoning one's purpose. Supporting characters include Nue, a Mikura who allies with Otoha, and Yurine, a spirit who serves as the Karas's guide and mentor. The series explores the moral complexities of power and duty, as Otoha battles not only Eko's forces but also his own inner turmoil.

The plot is driven by intense action sequences, blending traditional Japanese folklore with cyberpunk aesthetics, as Otoha and Eko clash in a series of high-stakes confrontations. The series delves into the origins of the Karas and the Mikura, shedding light on the ancient pact that binds them to the human world. As the final battle approaches, Otoha must fully embrace his role as the Karas to prevent Eko from erasing the boundary between worlds, a decision that carries profound personal and existential consequences. The story concludes with a resolution that underscores the cyclical nature of duty and the enduring struggle to maintain balance in a world where the supernatural and human realms intersect.