By A Slight Mistake summary: Sagara Mizuhime, who realized she had recollections of her previous life while wandering on the border of life and death, woke up on a hospital bed. And she was surprised that the environment surrounding her resembled the setting of 『Seventh Heaven』 which she unwillingly played before. Her role is the perfect young lady who happened to be the rival of the disappointing heroine 『Toujou Rin』. However, if observed carefully, there were a lot of slight differences from the game. The perfect young lady named Mizuhime became the perfect Prince called Beautiful Crossdresser and her one-year older brother turned five years older. Without walking along the route of attacking the disappointing heroine, Mizuhime began to choose a proper life. “Eh? Huh? Why did it come to this?” What will be the future of the girl who started to roll opposite the direction she wanted due to a slight mistake?