A story about a lonely samurai girl with "ONI" (demon) horns and her friend, as they bravely survive in a beautifully crafted world depicted through traditional Japanese ink wash paintings, amidst the chaos of war between humans and yokai.

Set in samurai-era Japan, tensions rise between humans and the powerful Empire of Spirits. Mao, a formidable samurai marked by mysterious ONI (demon) horns on her head, has known only loneliness after losing her parents and enduring a harsh upbringing by a tyrant. Her closest bond is with her best friend, Onatsu, whose love has been Mao's sole solace. As war erupts, Mao vows to protect both her homeland and Onatsu, but Onatsu declares, "This is the end of our friendship," leaving Mao heartbroken. The true meaning of Onatsu's words and the fate of their friendship lie in the mystery of Mao's horns.