Oscar appeared as a member of the secret Black Lilly Clique who wished to reinstate the old power of the pre-Revolutionary nobility in France, but without the monarchy. The reason for Oscar's appearance is left undisclosed; it is conceivable that she was a possible reincarnation of the Revolution era Oscar, reborn into a surviving bloodline of her old family.
Despite her membership in and her duties to that Society, Oscar longed only for one goal in her life: being reunited with her lost love André, a royal guard who died sometime before the onset of the French Revolution. The only surviving memorabilia of André was a statue at Normandy Beach, and the episode's plot suggests that this statue is in fact André himself, petrified by a secret potion after caused the wrath of Queen Marie Antoinette. Oscar is still loyal and doesn't want to disobey their late Queen's wishes, so instead of returning him to a normal state, she decided to turn herself into a stone and stay with him forever. However, the only surviving sample of the potion was contained in a diamond set into the crown of Marie Antoinette, which was kept at her old palace in Versailles inside a hidden vault.
In order to be reunited with André, Oscar contacted Lupin III and promised him Marie Antoinette's treasure if he would help her recover the crown. Lupin, who was to his own surprise attracted to Oscar even though he believed her to be a man, succeeded in doing so, but then Oscar betrayed him and the Society, stealing the crown from him as she was desparate to be together with André again. Lupin managed to recover and chased after her, eventually catching up as she tried to return to Normandy beach with an old biplane. However, the Society sent its own triplane squad to kill her for her betrayal. Lupin ended up saving Oscar from her pursuers and paying the Society back in kind, killing their leader, Jean.
Later, in front of André's statue, Lupin and Oscar fought a sword duel over possession of the crown. Provided with Goemon's Zantetsuken, Lupin defeated Oscar, thereby shredding her uniform and revealing her as a woman. Oscar accepted defeat, but asked Lupin for the diamond in the crown. Before Lupin, Goemon and Jigen realized what was going on, Oscar opened the jewel and swallowed the potion within, before she positioned herself to the side of André's statue. As her beautiful body completely naked slowly turned to stone, she finally explained her ultimate motives to a dumbfounded Lupin, before she finally died, petrified, with her eyes resting on André's face.