The Mitsurugi household has been entrusted by the emperor with the sacred duty of guarding the demon gate. Sora Mitsurugi, the heir of the family, reaches the age of thirteen and must undergo the trial ceremony to inherit the phantom blade style, a sword technique passed down through the Mitsurugi bloodline for generations. Seven other pupils of the same age have already passed the test, leaving Sora as the only one yet to prove himself. His father, brother, fiancée, and a pair of siblings who have watched over him all gather to witness the solemn beginning of his trial.

Sora fails the test. Branded as incompetent and deemed weak, he is expelled from the Mitsurugi family by his own father, the seventeenth sword saint and master of Onigashima, who believes there is no place for the weak. Stripped of his birthright and cast out, Sora tries to rebuild his life as an adventurer in the distant town of Ishuka, but even there he struggles to find footing and lives in misery. Five years after his exile, he is betrayed by former companions who abandon him to a monster known as the King of Flies. Left to be devoured alive, Sora is at death's door when a sudden, uncontrollable power surges through his body. He awakens as a soul eater dragon, a being capable of devouring the souls of others, gaining immense and seemingly invincible strength.

Empowered by this transformation, Sora vows revenge against those who discarded him. The story follows his path of retribution as he confronts the family that rejected him, the comrades who betrayed him, and the larger forces tied to the demon gate his family once guarded. Along the way, he encounters various allies and foes, and the manga follows his rise from a castoff heir to a vengeful force wielding the power of soul consumption. The narrative blends themes of betrayal, survival, and revenge within a dark fantasy world where swordsmanship and supernatural abilities collide.