Though I Am an Inept Villainess unfolds in the Kingdom of Ei's opulent inner court, where the thriving Maiden Court supplies candidates for the empress and the four imperial consorts from five prominent clans. Among them, the pristine consort-in-training Reirin Kou stands out as the perfect future empress. Her virtue and elegance are unrivaled, and she seems flawless save for her extremely sickly constitution. Opposite the court's gentle butterfly stands the boorish Keigetsu Shu, treated like a sewer rat and scorned as a disgrace to the Maiden Court even as she watches Reirin with bitter envy. As the maidens compete for favor around the Crown Prince, that jealousy turns deadly on the night of a rare comet.
Keigetsu pushes Reirin from a balcony, and although Reirin is quickly saved from death, Keigetsu is marked for execution. Unbeknownst to the court, however, Keigetsu has already cast a forbidden spell to swap bodies with Reirin in order to steal all that belongs to her. The plan does not unfold as she expects: Keigetsu is hindered by Reirin's frail body and left teetering on the verge of death, while Reirin embraces physical strength and freedom for the first time in her life, determined to live no matter whose face she wears. Forced to answer for Keigetsu's crimes, Reirin is drawn into rivalries between clans and darker conspiracies beneath the palace's polished surface.