The children of the Three Musketeers swing into action against a traitor.

In 1648, in the wake of Cardinal Richelieu's demise and the approaching death of the Queen, France is plunged into political chaos. The Queen's most power-hungry opponent is the Duke de Lavalle, who, as a leading member of the Council of Noblemen, is scheming to marry the Queen's daughter Henriette and kill the young Prince. Because Lavalle has murdered every royal messenger who has tried to deliver pleas for help to the King of Spain, the Queen sends her guards to find the Musketeers, four brave men who aided her when she was young. Unknown to the Queen, all of the Musketeers have either died or are too old to return to service. Three of the Musketeers--D'Artagnan, Aramis and Porthos--have grown sons, however, and the young men eagerly respond to the Queen's summons. The fourth Musketeer, Athos, sends his daughter Claire, an expert swordswoman, who dresses in men's clothing. When they all meet at the same inn at which their fathers used to rendezvous, the three young men assume Claire is a man and prepare to room with her. Terrified at the prospect, Claire suddenly lets down her long hair and orders the men to sleep in the stables. The next morning, some of Lavalle's men enter the inn looking for the Musketeers and, without identifying themselves, the new Musketeers challenge them to a sword fight. Later, at the palace, the Musketeers introduce themselves to the Queen, who is surprised but grateful. The Queen asks the Musketeers to escort Henriette to Spain, so that Spain can safely force the Council of Noblemen to reject Lavalle. The sickly Queen then reveals the location of the monastery at which her son has been hiding. Unknown to the Queen, her daughter's trusted lady-in-waiting, Countess Claudine, is a spy for Lavalle and is aware of the Musketeers' plan to abduct Henriette from Lavalle's men. Ordered by the Queen to return home, Claire pledges her love to D'Artagnan before she and the others head off in opposite directions. The three remaining Musketeers then descend on the carriage on which Henriette is traveling, but are trapped by Lavalle's men. Riding nearby, Claire joins the ensuing fracas and is arrested along with the other Musketeers. In prison, Lavalle tries to torture Porthos, Aramis and D'Artagnan into revealing the Prince's whereabouts, but they, and Claire, remain silent. Desperate to break them, Lavalle orders that the Musketeers be executed one by one, starting with D'Artagnan. Just before he is to die, however, Lavalle offers to spare the Musketeers if the Queen gives him permission to marry Henriette. The Queen accepts Lavalle's terms, and the Musketeers are freed. To Lavalle's shock, a veiled Claire takes Henriette's place at the wedding ceremony, while the other Musketeers whisk Henriette to the inn. There, they hear that the Queen has died. Dressed as a boy, Henriette then leaves with the Musketeers for the monastery, one step ahead of Lavalle's guards. The imprisoned Claire, meanwhile, is befriended by Claudine, who helps her to escape and, after declaring that the other Musketeers are dead, coaxes her into revealing the Prince's location. Before leaving with Claire, Claudine gets word to Lavalle, whose men race to the monastery, arriving ahead of the Musketeers. When Claire sees Porthos guarding the road to the monastery, she deduces Claudine's betrayal and alerts Porthos. Claudine is captured and taken to the inn, where she confesses Lavalle's plan to murder the Prince. To determine where Lavalle has taken the Prince, Claire poses as a seductive barmaid and lures one of Lavalle's guards into a trap. After the man reveals that the Prince is being held at Lavalle's heavily guarded castle, the Musketeers scour the countryside, soliciting aid from the peasantry. While the peasants prepare to storm the castle, the Musketeers convince Claudine that they are surrendering and allow her to deliver them to Lavalle. Moments before the castle is besieged, the Musketeers take Lavalle by surprise, while Claire grabs the Prince. During the ensuing fight, Lavalle takes the Prince hostage, but D'Artagnan outmaneuvers him and kills him in a sword fight. With the crown finally secure, the Musketeers pledge their loyalty to the new King.