Yami o Terasu Mono takes place at some undetermined point in the future in Vol City, a metropolis build around a volcano. One day, Ryūga Dōgai arrives in the city, drawn in by its high concentration of evil spirits known as Horrors, the ones plaguing Vol City being Madō Horrors. Ryūga is in fact a Makai Knight and the latest knight to bear the title of Garo the Golden Knight. However, Garo's armor has mysteriously been tarnished black. Leaving many questions behind the lost of his golden armor's radiance, and the fate of his predecessor, Kouga Saezima.