Laviange Robur, the first daughter of the noble House Robur, is widely considered the most disagreeable, irresponsible, and talentless young woman in high society. Her magical ability is nonexistent, her education is lacking, and her reputation is in shambles. Everyone believes she is utterly incompetent, but this is a carefully constructed facade. Laviange is living her third life. In her first life, she was the legendary villainess and genius mage Beljeanne. In her second, she lived as an ordinary woman in Japan, growing to the age of eighty-six, getting married, and raising a family. Now, reincarnated back into her original world as the granddaughter of her former rival, she carries the memories and wisdom of both lifetimes. Her only goal in this third life is to take it easy. By hiding her immense magical power and playing the part of an inept and harmless noble, she enjoys the freedom and peace that being underestimated provides.

The story is set in a fantasy noble society with magic, academy life, and complex family politics. Laviange must navigate the treacherous environment of her own household, which includes a mother who physically abuses her, an indifferent father, a priggish brother, and a manipulative adopted sister who acts as a seemingly virtuous heroine. Her plan for a quiet life falls apart during a mandatory outdoor training exercise for students of lower academic standing. When the exercise goes horribly wrong, turning into a life-threatening situation filled with powerful monsters, Laviange is forced to act. Drawing on the power of an unstoppable mage and the pragmatic senses of an old lady, she accidentally unleashes a rampage that decimates the threats and saves the group. This sudden display of power shatters her reputation as a useless noble and begins to transform her into someone both admired and feared.

A major narrative arc involves Laviange enacting precise and ruthless revenge against those who have wronged her. She systematically targets the arrogant and corrupt nobles who have bullied her or tried to harm her, such as a noble who used mind-control magic on her during the dangerous training exercise. Her vengeance is carefully calculated, directed only at those she deems deserving, while she remains genuinely kind and protective of allies who treat her with respect. The second prince, who tormented her and is being manipulated by her adopted sister, eventually realizes the gravity of his actions and attempts to atone, though his efforts are shown to be too little, too late. As Laviange navigates this new reality where her powers are no longer a secret, she must balance her desire for a peaceful, anonymous life with the consequences of her accidental rampage, all while the mysteries of her first life as the villainess Beljeanne slowly come back into focus.