The kingdom's hero, Ortus, dies in battle after failing to fulfil a promise he made to protect the kingdom for the rest of his life. When he wakes up, he finds that he has been reincarnated as Seretina, the duke's daughter! Even though she's a lovely, beautiful girl, she's still the same and determined to become a knight to protect her country in this life. However, besides being sickly and fragile, she is also faced with the harsh fate of a lady. She has to learn manners from her horrible mother, marry a man she doesn't want to marry, and many other challenges that will keep her from becoming a knight. Then, on the way to the royal castle, a demon attack reveals her true power that no one knows about! A new heroine is born, with the strength to slay demons with a single blow, and the peerless beauty to captivate even people in high society! She’s pretty on the outside, but fearless on the inside! The chivalric, heroic fantasy of a feeble young woman begins!...