Sister Alicia Snowell leads a double life. By day, she is a devout bride of the gods serving the Holy Church. By night, she is one of the Churchs top inquisitors, an assassin who eliminates heretics without mercy. Her newest mission is her most dangerous yet: kill the Hero Elcyon, the celebrated slayer of the Demon Lord. The Heros growing fame threatens to eclipse the Churchs political power, but he is said to be divinely protected and unkillable by normal means. The only way to succeed is to first win his heart.

Alicia sets out to seduce the Hero, only to discover a critical flaw in the plan. Elcyon is not a man but a young woman named Cion. Thrown into confusion, Alicia must now figure out how to romance a girl while hiding her lethal intentions. As she struggles with this unexpected complication, remnants of the demon army launch attacks, forcing Alicia and Cion to fight side by side in deadly supernatural battles.

The story follows Alicia as her cynical, bloodstained worldview collides with Cions sincere nature. Behind her bitter exterior, Alicia is driven by repressed grief and a deep-seated care for others that she cannot suppress, leading her to make increasingly difficult choices.

The first major arc focuses on their initial encounter and a battle against General Heavenfang, a former demon army commander. Victorious, they are then dispatched to the Holy City of Eldias. A mysterious killer is murdering cardinals, and the two are assigned to protect Saint Nevissa Vernalia, the Blind-Eyed Saint known for her kindness toward demon-touched children. There, a shadowy figure named Veil Croitzen contacts Alicia, claiming to be the killer and accusing the saint of being a demon in disguise. When Alicia tries to uncover the truth, she finds an unexpected obstacle standing in her way: Cion herself, who has become strangely charmed by the saint.

As the series progresses, Alicia faces the death of her mentor and is pushed to the brink of her morality, almost turning evil before pulling back. The narrative explores the nature of demons and humans, forcing the pair to confront new truths that threaten to shatter their budding relationship.