The story begins with Diara, a young girl who appears to be around five years old but is actually fifteen, standing in a stone tower. She is about to perform a summoning ritual to bring forth the Demon Lord Hyberumoot, whom she intends to make her servant. Diara possesses memories of her past life, where she was a renowned magic researcher in the Kingdom of Grendel but was assassinated. Reincarnated as a child, she found her magic power weaker than before. Driven by a desire to live as she pleases without restrictions, she decided to summon a powerful demon to compensate for her magic.
At the age of five, Diara infiltrated the Royal Central Church's ancient document library, stealing a forbidden tome. In a secret ruin she calls her hideout, she deciphered the book, which contained methods to summon legendary demons from the Age of Gods, including Demon Lords. For ten years, she diligently poured her magic into a summoning circle she drew with her own blood, even sacrificing her own growth to empower the ritual.
As the ritual reaches its climax, a black mist appears, taking the form of a kneeling figure. A long sword and golden horns are glimpsed, confirming it is the Demon Lord Hyberumoot. Diara initiates the contract spell, seeking a temporary marriage with the demon to gain its clan's power. She declares her name as Diara, and the demon, in a low voice, reveals its name to be "Sais." This name triggers a fleeting memory of a familiar man in Diara's mind. The contract is sealed, and the tower fills with light.
When the light subsides, Diara finds not a demon, but a man with gray hair and blue eyes, clad in knight's attire and carrying a sword. He is none other than King Lucis Audal of Grendel. Diara is shocked, realizing her summoning ritual went awry. A magical mark, resembling intertwined vines, appears on her left hand, signifying the marriage contract. Sais, equally bewildered, discovers the same mark on his hand.
Through their conversation, they deduce that Diara summoned Sais instead of Hyberumoot, and due to the completed contract magic, they are now bound by a marriage that can only be broken by death. Sais, now Diara's servant, is initially skeptical of her commands. However, when Diara orders him to do a handstand, he complies, confirming his subservience. Diara, frustrated that her ten years of effort have led to this unexpected outcome, dismisses Sais, telling him to return to the palace on his own.
Sais insists they go to the palace together, as he must publicly acknowledge their marriage. Diara vehemently refuses, but Sais suggests that by investigating the ancient tome at the Royal Palace's library, they might find a way to break the contract and allow her to summon other demons. The allure of the Royal Palace library, a repository of knowledge, entices Diara, and she agrees to go.
Upon arriving at the palace, Sais introduces Diara to his prime minister, claiming she is his destined wife whom he met in town. Diara's parents are shocked but ultimately agree to let her go with the King. Diara, still appearing as a five-year-old, plays along, hiding her true age and past life memories. She learns that the Grendel royal family has a rule requiring partners to be at least six generations removed to prevent the over-concentration of magic.
Inside the palace, Diara is shown the portrait of the previous queen, Frandellina, who was assassinated at seventeen. Diara reveals that she is Frandellina, her past life. She recounts how her uncontrolled magic, possibly due to inheriting the "Demon Lord Hyberumoot" upon her ascension, led to her assassination. She also recalls her childhood friendship with Sais, who was her cousin and later her vice-captain of the Royal Guard. Sais kept Frandellina's room preserved as a reminder of his failure to protect her.
They discover that the ancient tome Diara stole is the "Secret Tome of Vainveldt," the founder of Grendel. Within it, they find a summoning circle identical to the one Diara used, revealing that Vainveldt was actually the Demon Lord Hyberumoot, who was bound by the sage Noll to become the human king of Grendel. This revelation explains why Sais, a descendant of Hyberumoot, was summoned. The contract between Diara and Sais is broken when Hyberumoot's power is returned to the tome.
Diara, now free from the contract and her past life's regrets, returns to her parents. A year later, she has grown significantly and is reunited with Sais. He confesses his enduring love for her, revealing he had always loved Frandellina and waited for her reincarnation. Despite the age difference, they decide to marry, with Diara embracing her desire to live a truly selfish life. The story concludes with Diara and Sais embarking on a picnic, their relationship blossoming, and the promise of a future together.
