During the late 16th century, Catholics and Protestant Huguenots are fighting over political control of France, which is ruled by the neurotic, hypochondriac Charles IX of France (Jean-Hugues Anglade), and his mother, Catherine de' Medici (Virna Lisi), a scheming power player. Catherine decides to make an overture of goodwill by offering up her daughter Margaret of Valois (Isabelle Adjani) in marriage to Henry IV of France (Daniel Auteuil), a prominent Huguenot and Kingdom of Navarre, although she also schemes to bring about the notorious St. Bartholomew's Day Massacre of 1572, when thousands of Protestants are slaughtered. The marriage goes forward but Margot, who does not love Henri, begins a passionate affair with the soldier Joseph Boniface de La Môle (Vincent Pérez), also a Protestant from a well-to-do family. Murders by poisoning follow, as court intrigues multiply and Queen Catherine's villainous plotting to place her son the Duke of Anjou (Pascal Greggory) on the throne threatens the lives of La Môle, Margot and Henri of Navarre. A book with pages painted with arsenic is intended for Henri but instead causes the slow, agonizing death of King Charles. Henri escapes to Navarre and sends La Môle to fetch Margot, but Guise apprehends him. La Môle is beheaded in the Bastille before Margot can save him, and King Charles finally dies. Margot escapes carrying La Môle's embalmed head as Anjou is proclaimed King of France as Henry III of France.