Paris, 1626. The young, inexperienced King Louis XIII sits on the throne of France, but the actual power rests in the hands of Cardinal Richelieu. Meanwhile, in a hamlet in Southern France, the young, idealistic D' Artagnan saddles his horse and heads to Paris. It is his greatest dream to become, just like his father, a musketeer and member of the king's elite corps.

On the way he ends up in an inn - his first acquaintance with the 'wide world'. He sees there how the Hugenots (Protestants) and the Roman Catholics interact. The French Hugenots are supported by Protestant England, which threatens invasion if Richelieu does not leave them free to practice their faith. The tension between the two nations increases.

In the very same inn, Rochefort, the evil accomplice of Cardinal Richelieu, has an appointment with Milady de Winter. This femme fatal is a spy at the British court for the cardinal. Richelieu is the only one that can help her get rid of the fleur-de-lys - the brand on her right shoulder that was her punishment as a whore. She reports her latest findings to Rochefort.

In Paris, D'Artagnan becomes acquainted with the three musketeers: Athos, Porthos and Aramis. The three knew his father and are impressed by the striking resemblance. They take him into their circle. D'Artagnan also meets the sweet Constance. She is a lady-in-waiting for the queen Anne, the wife of Louis XIII. D'Artagnan quickly falls in love.

In the meantime, Richelieu sets out to isolate and attack the Hugenots. He persuades Queen Anne to write to Lord Buckingham, her former lover and now prime minister of England. Buckingham must withdraw his support of the Hugenots. The King is unaware of all of this.

Buckingham comes - incognito - to Paris. He still loves Anne, and she loves him. Nonetheless, she does not want to give in to her feelings. She has accepted her new role as queen of France. Yet, to affirm their solidarity, she gives Buckingham her diamond chain which constitutes part of the crown jewels. Milady secretly witnesses this, and immediately passes it on to Richelieu. He sees it as an opportunity to descredit Anne. He suggests that the King to throw a large ball in honour of his birthday. Naturally, the royal couple must wear all the crown jewels. Anne will be discovered as a traitor, and the furious king will give Richelieu permission to deal with the Hugenots.

Anne panics, and takes Constance into her confidence. She asks D'Artagnan to go to England and get the diamond back. D'Artagnan sets off, accompanied by his three friends, but he loses them all one by one along the way, due to Rochefort's intrigues. D'Artagnan meets up with Buckingham. He returns the diamond reluctantly to D'Artagnan, and realizes at the same time that it is his duty to support the Hugenots. D'Artagnan is conflicted: his mission is a success, but the outcome is war.

On the way back, Milady tries, with ruse and seduction, to steal the diamond, but D'Artagnan manages to escape. Milady is upset, for now she will never get rid of her stigmatising brand.

Richelieu considers a last trick: he sends Constance to a convent. He will release her if he gets the diamond. Milady is no longer needed for his plans, and he tries to put her off. The embittered Milady directs her rage at Constance, whom she poisions. D'Artagnan, reuinted with his three friends, is only in time to hold the dying Constance in his arms. The three musketeers proclaim the death sentence over Milady.

Meanwhile, the ball has begun. Richelieu still tries to succeed, by returning the diamond with great ceremony, but D'Artagnan gives it directly to Anne. Richelieu is unmasked, D'Artagnan's courage has been proved, and his dream is realized: the king appoints him as a musketeer.