A black knight named Lina is a powerful warrior capable of slaying dragons, but her career and her life depend on possessing old magic, which must be shared with a compatible partner. As a requirement for her dangerous role, she enters a contractual marriage with Joshua Granwell, a talented white mage and the assistant to the prime minister. Despite being a perfect magical match, their union is one of convenience rather than love, and Lina harbors a strong reluctance to marry a noble.

Four years into their strained marriage, Lina suffers grievous wounds during a dragon hunt because Joshua's protective magic proved inadequate, leading to her dismissal from her post. The story then traces the collapse of their relationship, revealing how the couple's inability to communicate, combined with the prejudice and disdain of a nobility that despises commoners like Lina, pushed them to the brink. Their suffering is eventually revealed to be the result of an engineered conspiracy by a figure from Joshua's past at the academy, who manipulated events to destroy their marriage and halt social reforms within the kingdom.

Following their divorce, the narrative moves to a remote borderland, where the former couple must confront the truth of their failed marriage and decide if they can rebuild a relationship as true partners. The story follows their journey from a purely contractual arrangement through bitter estrangement and loss, ultimately exploring whether they can learn to communicate honestly and reignite their romance without the expectations and pressures of their former positions. The tale is set in a fantasy kingdom where magic compatibility determines the safety and effectiveness of those who fight monsters, and where rigid social hierarchies create powerful obstacles for commoners who rise to noble rank.