The film opens with d'Artagnan being shot and buried alive after attempting to stop a violent night ambush on a noblewoman. He clawed his way out of the dirt and traveled to Paris. After his duels are interrupted by Richelieu's guards, d'Artagnan allies with Athos, Porthos, and Aramis.
The plot turns when Athos wakes up in a tavern room after a heavy night of drinking, finding a murdered woman beside him with his dagger in her chest. Arrested and sentenced to beheading, Athos refuses to flee, choosing death over dishonor. D'Artagnan discovers that the murdered woman was an agent linked to a conspiracy involving Cardinal Richelieu and Count Chalais.
The central conspiracy centers on Queen Anne of Austria, who has given her twelve diamond studs to her lover, the English Duke of Buckingham, as a secret token. Cardinal Richelieu and Milady de Winter manipulate King Louis XIII into demanding that Queen Anne wear those exact diamond studs at an upcoming royal ball, intending to expose her treason and force France into war with England.
D'Artagnan races across France to England, surviving assassination attempts led by Milady. He secures the diamonds from Buckingham and arrives back at the French court seconds before the Queen's absence is noticed, thwarting Richelieu's trap and saving Athos from the executioner's axe.
The Ending Cliffhanger: While celebrating his victory and his romance with Constance Bonacieux (the Queen's confidante), tragedy strikes. Constance overhears key conspirators discussing a secondary plot and is abruptly kidnapped from her quarters. D'Artagnan charges after her captors into the dark streets of Paris, but he is ambushed, struck on the head, knocked unconscious, and thrown into a carriage—leaving his fate and Constance's rescue unresolved until The Three Musketeers: Milady.