Lydiana "Lidi" von Vivoir is the daughter of a preeminent duke, but in her previous life, she was a normal Japanese woman. Carrying on the sentiments from that life, she wants to avoid marrying into the royal family, members of which are allowed to practice polygyny—the practice of having multiple wives. However, when she discovers that her father arranged her marriage with Crown Prince Friedrich "Freed" van de la Wilhelm, she thinks of a way to avoid her fate.

Lidi's solution is simple: lose her virginity, which is the one requirement for becoming the crown princess. Donning a mask, she goes to an illicit masquerade ball, where she encounters a mysterious, black-haired man. After spending the night together, she returns home, believing her mission complete, only to face the harsh reality soon after. The man she lost her virginity to is none other than the crown prince himself, and he is dead set on keeping her all to himself!