The kingdom of B, facing political pressure from its powerful neighbor, decides to send its fifth princess, Aruna, to marry the king of E Kingdom. Aware of the dangers that await, Aruna's father devises a plan to protect her. She swaps identities with her look-alike maid, Camilla. The princess takes on the role of a knight named Sumire, while Camilla becomes the decoy bride. Upon arriving at E Kingdom, they discover the situation is far worse than imagined. The court is a viper's nest, with five other women already competing to become the sole queen, a brutal struggle known as the Cold Game.

The setting is a fictional, medieval-inspired world where nations are referred to by letters, creating an atmosphere of cold political calculation. E Kingdom, in particular, is described by its own people as a "demon world," a place where kindness is a weakness and betrayal is commonplace. Aruna, raised in the peaceful B Kingdom, is horrified by this reality but quickly learns she must adapt to survive.

The main character is Aruna, who operates under the name Sumire. She is a kind but strong-willed princess with silver hair and violet eyes, trained in swordsmanship, which becomes essential for her survival. Her false identity as a knight puts her in close contact with King Arthur, the young and calculating ruler of E Kingdom. Initially hiding his identity, Arthur is intrigued by Sumire's fearless nature and appoints her as his attendant. Camilla, the gentle and timid maid, is ill-suited for her role as the fake princess and suffers immense psychological pressure from the court's cruelty. Other key players include Anne Gregory, a seemingly serene queen candidate who is ruthlessly ambitious and responsible for many of the court's murders, and her enigmatic sister Christina. Edward Hughes, Arthur's sharp-tongued but loyal retainer, becomes an ally to Sumire.

The story unfolds through several major arcs. The early narrative focuses on the initial shock of the court's brutality. Sumire befriends another candidate, Lizzy, only to witness her being framed and executed by Anne Gregory. This event hardens Sumire's resolve, but the real turning point is the death of Camilla. Broken by the court's horrors, the decoy princess is found dead, transforming Aruna's desire to survive into a fierce vow for revenge. As Sumire, she begins to actively fight back, participating in the political and physical conflicts of the court.

The middle portion of the story expands beyond the palace walls. As Sumire proves her worth as a knight, she and King Arthur develop a complex bond built on mutual respect, moving beyond a simple master-servant relationship. The narrative escalates from a palace drama into a full-scale war when E Kingdom is threatened by northern tribes. On the battlefield, Sumire earns a legendary reputation as a "war maiden". This arc also delves into Arthur's tragic past, revealing how he was forced to execute his beloved mentor, a man named Vallie, by his own treacherous brother, an event that shaped his cold persona.

In the most recent narrative arcs, the story becomes a hunt for answers. A mysterious figure identical to the supposedly dead Vallie appears, targeting Arthur and reopening old wounds. This mystery forms the core of the current conflict. Meanwhile, back in the capital, the fight for the throne intensifies. Having risen to the rank of vice-commander, Sumire finds herself at the center of new political schemes. A love triangle begins to form as Edward openly declares his feelings for her, creating tension between his loyalty to the king and his love for Sumire, who only wishes to remain by Arthur's side as a knight. As of the latest chapters, the truth about Vallie and the future of Arthur and Sumire's relationship remain the central, unresolved questions of this ongoing survival game.