Jeanne de Valois-Saint-Rémy, a determined noblewoman who lost her family's title early on, embarks on a daring quest to reclaim her lineage and retrieve the ancestral estate taken from her. After being met with rejection from Marie Antoinette herself and failing to achieve her aim through formal means, Jeanne enlists the help of Rétaux de Villette, an astute socialite, alongside her rebellious husband, Nicholas. Together, they devise a scheme to amass enough fortune to buy back their lost property.
The narrative takes a fascinating twist when it is revealed that King Louis XV had commissioned Boehmer & Bassenge, leading jewelers of Paris, to craft a stunning 2,800-carat diamond necklace for his mistress, Madame du Barry. However, the king's sudden death before the necklace's completion left the jewelers reeling from considerable financial setbacks. In their quest to recover their losses, they turn to Queen Marie Antoinette, only to be denied due to her awareness of the necklace's troubling history.
Seizing this turn of events, Jeanne, a skilled strategist, manipulates Cardinal Louis de Rohan, a debauched libertine desperate for the Queen's favor and the position of Prime Minister. By masquerading as one of Marie Antoinette's trusted advisors, she easily ingratiates herself with the Cardinal, convincing him that she holds the key to rekindling his relationship with the Queen. With the counsel of the enigmatic occultist Count Cagliostro, the Cardinal becomes increasingly convinced that he is receiving special attention from the Queen.
As their correspondence deepens, Jeanne orchestrates a secret meeting between the Cardinal and Marie Antoinette in the opulent gardens of Versailles. The role of the Queen is portrayed by Nicole Leguay d'Oliva, a courtesan who resembles her majesty. Cloaked in secrecy, she consents to forgive past grievances. The Cardinal, completely consumed by his desires, thinks he has regained the Queen's affections, unaware that his correspondence is being manipulated through a series of forged letters, perpetuating a web of deceit.
With a deft hand, Jeanne guides Cardinal de Rohan in his quest to obtain the magnificent necklace supposedly on the Queen's request, cleverly shrouding her true motives behind a mask of misdirection. The Cardinal, blind to Jeanne's cleverness, willingly fulfills her demands and presents the jewel to Rétaux de Villette, who is misled into believing he represents Marie Antoinette. Meanwhile, Nicholas seizes the opportunity to sell some of the exquisite diamonds, generating a tidy profit that Jeanne then uses to reclaim her family estate.
However, the Cardinal's world is thrown into disarray when Jeanne mysteriously disappears and all communication with the Queen halts. Nicholas narrowly escapes arrest for selling without the appropriate documentation, only to receive advice from Jeanne to halt all diamond dealings in Paris. She subsequently sends letters to the jewelers, stating that Marie Antoinette has lost interest in the necklace and demanding repayment from the Cardinal. Unfortunately, Minister Breteuil intercepts a troubled Boehmer on his way to the Cardinal's estate, initiating a series of events that ensnare all parties involved.
As the Feast of the Assumption draws near, the Cardinal awaits his promised repayment and hopes to be named Prime Minister by Marie Antoinette. Instead, King Louis XVI, informed of the Cardinal's schemes through Breteuil, orders his imprisonment in the notorious Bastille. The fallout is profound, leading to Rétaux de Villette's conviction and subsequent banishment from France. Jeanne faces trial, subjected to brutal whipping and branding before her imprisonment, ultimately escaping to London where she writes her memoirs, entrancing the local populace with tales of her exploits.
The unfolding story closes with an epilogue, revealing the tragic fate that awaits Marie Antoinette at the guillotine. In a dramatic twist, Breteuil divulges that Jeanne never returned to France, meeting her fate after a fall from her hotel room window, with rumors suggesting that Royalists were behind her tragic end.
