I Heard My Lovely Sister-In-Law's Engagement Was Broken So I'm Going to Show Them My 'Thanks!'Kawaii Gimai ga Konyakuhaki Saretarashii no de, Ima kara "Orei" ni Mairimasu; 可愛い義妹が婚約破棄されたらしいので、今から「御礼」に参ります。; I Heard My Lovely Sister-In-Law's Engagement was Broken So I'm Going to Show Them My "Thanks!"

Admired by everyone, Rozelia is the country's most talented individual, accomplished in both the literary and military arts. Today is her wedding with her childhood friend and lover, Robert, who is to be the successor of a duchy. Everyone wished them well, and they were supposed to spend the rest of the happy lives together, however... that night, Robert's sister, Margaret, arrives in the mansion in tatters! Margaret was the crown prince's fiance, but she was suddenly falsely accused of a crime, had her engagement broken, and suffered terrible violence. Rozelia was greatly angered at the fact that the selfish prince would treat Margaret in such a shameful manner despite Margaret being the one to have cleaned up all the mistakes he caused. For the sake of her beloved sister-in-law, Rozelia decides to cut up the corrupted royal palace straight from the front!