Kashimir grew up working as a mercenary to buy medicine for her precious younger half-sister, Aria. On her way home, after completing a mission as usual, she tripped and hit her head, causing her to remember her past life; thus she realized she had been reborn in the world of the novel The Night of the Fairy. Her sister was the main character of this reverse-harem romance while she was one of the villains causing Aria to suffer hardships. Since there is no way Kashumir would let her precious sister suffer, she vowed to protect Aria until her adoption into the count's family, just like in the novel. But things turned out differently, and even the male lead candidates, who should've been fish in Aria's pond, all ended up becoming obsessed with Kashumir instead, when all she wanted was to protect her lovely sister. To save her younger sister Aria from her illness, Cassmire Crisis became a mercenary, wielding a sword and shedding blood to pay for her medicine. But when Cassmire suffers a fall one day, memories of her past life as Aria’s wicked stepsister come flooding back. Rather than take up the role of villainess again, Cassmire vows to protect Aria this time. But with Aria doomed by the story to face only hardships, Cassmire must pick up her weapon once more to become her sister’s loyal sword!