In a fantasy kingdom where magical ability determines social standing, Thetis is the overlooked younger daughter of a count. Born into a prestigious family of barrier magicians, she possesses only a faint spark of magic and is dismissed as useless by her parents and society, who constantly compare her to her brilliant older sister, Hilda. Hilda is beautiful, talented, and adored by everyone, while Thetis has long resigned herself to a life of neglect.

Thetis's world is turned upside down when a marriage proposal arrives from Noah Savoid, the eighteen-year-old Duke and the nation's most powerful chief magician. Bewildered as to why someone of his stature would choose her, she is given a cruel explanation by her sister: Noah is actually in love with Hilda, but Hilda already has a fiancé. Thetis is to go as a substitute bride, a replacement to console him. Dreading a life of cold formality as a stand-in, Thetis prepares to move into the Savoid household with no expectations of happiness.

Upon her arrival, however, her expectations are shattered. Noah greets her not with cold indifference, but with warm, gentle eyes and overwhelming affection. Far from treating her as a substitute, he showers her with kindness and adoration, insisting he has been eagerly awaiting her arrival. Thetis, who has never been valued before, is utterly confused by his sincerity, especially given her sister's claim about his true feelings.

As the story unfolds, Thetis must navigate her new life as a duchess and her baffling relationship with Noah. While she struggles to understand his motives, she also pursues a long-held personal dream: to become a powerful barrier magician like the ancestors of her family line. A recent, unexplained phenomenon has allowed some people to increase their magical power, and Thetis hopes to unlock that secret. Her journey intersects with her new husband's responsibilities, including investigations into magical disturbances, such as a water dragon that has gone berserk in a lake in the Marelia territory. During this mission, they meet Chloe, a viscount's daughter who is a childhood friend of Noah, testing Thetis's fragile confidence.

The narrative follows Thetis's transition from a disregarded, low-magic noblewoman into a confident and skilled barrier magician, all while she slowly uncovers the truth behind Noah's seemingly inexplicable love for her. The central conflict revolves around whether the love Noah shows her is genuine or a result of her being a substitute for her sister, and whether she can overcome her own deep-seated insecurities to claim her own worth and power.