Anne d'Autriche reigns as Queen of France beside King Louis XIII. Born a Spanish princess to King Philip III of Spain, her marriage to Louis XIII cements an alliance between France and Spain. Though a Spanish Habsburg, she carries the dynastic title "of Austria".

At the French court, her Spanish roots render her politically vulnerable, especially when France goes to war with Spain. Suspicions arise that she secretly corresponds with her brother, King Philip IV of Spain, using go-betweens like the Duchess of Chevreuse and Spanish ambassadors. Cardinal Richelieu's investigations prove her engagement in covert communications, resulting in constraints on her movements.

A pivotal scheme revolves around her gift of diamond studs to the Duke of Buckingham. Cardinal Richelieu seizes upon this to damage her standing with the king. Richelieu engineers a situation where Louis XIII commands her to wear the studs at a court function, aware that their absence would suggest disloyalty. The Musketeers race to recover the studs from England and replace lost ones, safeguarding her honor.

Amidst the court's treacherous politics, she exhibits resilience, depending on allies such as the Musketeers to shield her from Richelieu's plots. Her marriage to Louis XIII suffers from years without an heir and deep distrust, yet they ultimately produce two sons.

Historically, she acted as regent for her son Louis XIV after her husband's demise. However, the anime focuses on her earlier challenges: preserving her influence and dignity in the French court while balancing loyalties between her native Spain and her adopted France.