Jeanne de Saint-Rémy de Valois, orphaned at an early age, is determined to reclaim her noble title and the home taken from her family when she was a child. When she is rebuffed by Marie Antoinette and fails to achieve her goal through legal channels, she joins forces with the arrogant, well-connected gigolo Rétaux de Villette and her own wayward, womanizing husband Nicholas. They concoct a plan to earn her enough money to purchase the property.
In 1772, King Louis XV had commissioned Parisian jewellers Boehmer & Bassenge to create an opulent 2,800-carat (560 g), 647-diamond necklace to present to his mistress Madame du Barry, but the king died before it was completed. Hoping to recover the high cost of the necklace, its creators try to persuade Queen Marie Antoinette to purchase it. Knowing its history, she declines.
Jeanne approaches debauched libertine Cardinal Louis de Rohan and introduces herself as a confidante of the Queen. For years the Cardinal has yearned to regain the Queen’s favor and acquire the position of Prime Minister of France, and when he is reassured by occultist Count Cagliostro that Jeanne is legitimate, he allows himself to be seduced by her promise to intervene on his behalf. He begins to correspond with the Queen and is unaware that his letters to her are intercepted and the Queen’s responses are forgeries intended to manipulate him. The tone of the letters becomes very intimate. The cardinal becomes more and more convinced that Marie Antoinette is in love with him, and he becomes ardently enamored of her.
Jeanne allegedly arranges a meeting between the two in the gardens of the Palace of Versailles. Portraying the Queen is Nicole Leguay d’Oliva, a prostitute bearing some resemblance to her. Heavily cloaked, with her face in the shadows, she agrees to forget their past disagreements. The Cardinal believes his indiscretions have been forgiven and he once again is in the Queen’s good favor.
Jeanne advises the Cardinal the Queen has decided to purchase the necklace but, not wanting to offend the populace by openly buying such an expensive trinket, she wishes him to do so on her behalf, with a promise to reimburse him for the cost by the Feast of the Assumption. The Cardinal gladly agrees and presents the necklace to Rétaux de Villette, believing him to be an emissary from the Queen. Nicholas de Lamotte sells some of the diamonds, and Jeanne uses the profits to buy her family home.
The Cardinal begins to panic when Jeanne disappears and his correspondence with the Queen comes to an abrupt end. Nicholas is almost arrested for selling without proper certification, but he escapes. Jeanne advises him to not sell anymore diamonds in Paris. She sends correspondence to the jewelers, saying that Antoinette is no longer interested in the necklace and they must ask the Cardinal for reimbursement. However, Minister Breteuil comes upon an anxious Boehmer on his way to the Cardinal’s estate. The Cardinal is invited to visit the palace on the Feast of the Assumption, at which time he assumes he will be repaid in full and named Prime Minister. Instead, King Louis XVI, who has been made aware of his machinations by Minister Breteuil, has him imprisoned in the Bastille. Soon to follow are everyone else involved in the plot, excluding Nicholas who fled to the boarder and into Austria. A trial finds the Cardinal, Count Cagliostro, and Nicole Leguay d’Oliva innocent of all charges. Rétaux de Villette is found guilty and banished from France. Jeanne is found guilty and whipped and branded before being imprisoned; she later escapes to London where she publishes her memoirs and regales the locals with her tales. Eventually, Marie Antoinette, assumed to be a key player in the affair by an increasingly angry and restless populace, meets her fate on the guillotine. Via an epilogue, Breteuil explains that Jeanne never returned to France as she died after falling from her hotel room window and was rumored to have been killed by Royalists.