Claire is a diligent magical pharmacist who works in a knight order's medical division while being mistreated by her own family. Her beautiful and cunning younger sister, Scarlet, is the only one who receives affection from their parents, while Claire is treated as a burden despite her hard work funding the household. The one bright spot in her life is her kind fiancé, Oscar, who offers her gentle words of encouragement.

Scarlet devises a cruel scheme to take everything from Claire. She weeps false tears, accusing Claire of bullying her out of jealousy, and the family eagerly believes the lies, condemning Claire as having a wretched personality. Scarlet forces Claire to brew a powerful love potion, which is then unknowingly administered to Oscar. When Claire discovers that her own fiancé has been enchanted by the potion she was coerced into making, she is devastated. With nowhere left to turn, she is driven out of her home with nothing but the clothes on her back, and she unilaterally resigns from the knight order, unable to bear the sight of her former beloved.

Alone and at the lowest point in her life, Claire leaves her homeland behind. She journeys to a neighboring country and begins working as a caretaker for a man known as the Flame Knight, a reticent knight named Frederick who carries a troubled past. Frederick once earned his legendary title for his prowess in battle, but he is now burdened by a curse he received from a sacred beast of Rotania. The curse was a punishment for his past sin of destroying a refugee camp with his own hands. Claire, who has hidden her true identity and her history as a pharmacist, starts working in his household not as a healer, but as a maid.

As Claire settles into her new life, she and Frederick, both wounded by their pasts, slowly begin to open their hearts to one another. However, their fragile peace is shattered when a survivor from the destroyed refugee camp appears seeking revenge. In the ensuing battle, Frederick fights back, but his curse goes out of control and he loses consciousness. Seeing him fall, Claire makes a fateful decision. She chooses to use a forbidden medicine known as the Witch's Secret Potion that she had prepared for him. The potion is her only hope to save his life and atone for her own past, but its effects come at a terrible price, forcing Claire to confront not only Frederick's suffering but also her own long-suppressed desire to forgive herself and find true happiness.