Jiro Azuma is a high school student with an ability as absurd as it is dangerous: he can understand and speak the language of animals. Descended from a lineage of ninja, Jiro is skilled in hand-to-hand combat, but his life takes a drastic turn when he encounters Rago, a mortally wounded Mononoke (a demonic spirit) who resembles a small black cat. Rather than turn away, Jiro decides to help and protect him, but his choice comes at a cost: an enemy Mononoke attacks them and stabs the boy in the heart. To save his life, Rago takes the extreme step of fusing his soul with Jiro's. The result is the birth of a hybrid entity endowed with terrifying and destructive power. This event immediately attracts the attention of the Bureau of Ninja Intelligence, a secret government organization specialized in hunting down evil spirits that threaten the human world. Forcibly recruited into the "Black Torch" team, Jiro finds himself caught in a ruthless war against monstrous creatures and age-old conspiracies, forced to rely on the demon bound to his heart and his own fists to avoid being crushed by the darkness.