Rua Restoatt is a candidate for the rank of Saintess in the kingdom of Zied, a fantasy realm where a legendary healer once saved the nation from plague and is now revered as a goddess. Rua, the orphaned daughter of a baron, was raised in the Temple and possesses genuine healing powers, but her ability carries a devastating flaw: she can only cure others by transferring their injuries and ailments onto her own body. Because of this, she is mocked and belittled by the other candidates and Temple inhabitants, who call her the Broken Saintess or Fallen Saintess. Her only seeming supporter is Arianne Trebias, whom she considers her best friend.

When Garrett Vance, the captain of the Royal Knights and Rua's secret crush, is critically wounded in battle against a demonic beast, Rua pours all her strength into healing him, nearly dying in the process. Upon awakening, she discovers that Arianne has taken full credit for saving Garrett, claimed the miracle as her own, and is now engaged to him. Arianne is also being elevated to the vacant position of Saintess, a title that has remained empty for ten years following the previous Saintess's death. Crushed by the betrayal and further mistreated by the people around her, Rua vows never to heal again. In her despair, she discovers that she possesses a hidden second ability: she can inflict the very pain and suffering she once absorbed back onto others. She begins using this power to hurt those who wronged her, reveling in their agony.

Rua's actions catch the attention of Sven Geed Crone, the second prince of Zied. Sven, himself a prince with a past of being treated as an outcast, approaches Rua and proposes a partnership. He reveals that the Temple's High Priest is secretly engineering an artificially created, near-incurable plague to unleash upon the kingdom. The plan is to have the Temple's publicly sponsored Saintess, Arianne, heal the infected, thereby cementing the Temple's power and popularity, increasing the High Priest's own influence, and paving the way to seize control of the kingdom. After witnessing a clandestine meeting between the High Priest and Arianne that confirms Sven's accusations, Rua agrees to join his plan. She decides to use her newfound sadistic power, her knowledge of the Temple, and her alliance with Sven to exact revenge on everyone who betrayed and belittled her—Arianne, Garrett, the High Priest, and the system that used her—while helping the prince expose the corruption and stop the conspiracy before it can succeed. The story follows Rua as she transforms from a self-sacrificing healer into a calculating villainess, walking a path of vengeance, political intrigue, and dark romance, where her hidden power and her ties to Sven become the weapons she uses to dismantle the very institution that made her suffer.