Athletic adaptation of Alexandre Dumas' classic adventure about the king's musketeers and their mission to protect France.

In 1625, D'Artagnan, an eager young swordsman living in the small village of Gascon, France, sets out for Paris to seek a better life. In Paris, D'Artagnan meets Treville, the captain of King Louis XIII's musketeers, and asks to join Aramis, Porthos and Athos, the king's best musketeers, by whom he had been good-naturedly fooled on the road to Paris. Treville initially rejects D'Artagnan's request, but D'Artagnan later secures his place with the elite musketeers after fighting a series of duels and proving himself to be an expert swordsman. Soon after joining the musketeers, D'Artagnan learns that Cardinal Richelieu, the king's first minister, is secretly betraying the king and is determined to wage war against England. Treville is aware of Richelieu's betrayal but warns D'Artagnan that the king's position is tenuous and that D'Artagnan must not humiliate or provoke Richelieu's men. One day, Monsieur Bonacieux, D'Artagnan's landlord, asks D'Artagnan to look after his goddaughter Constance while he is away on a journey. Bonacieux tells D'Artagnan that Constance, who is one of the queen's maids, is in constant danger and that she is being followed by Richelieu's men. Before leaving on his journey, Bonacieux shows D'Artagnan a secret door in the apartment floor through which he may spy on Constance. When D'Artagnan sees Constance for the first time, he falls instantly in love with her. Late at night, after hearing screams from the apartment below, D'Artagnan dashes downstairs to rescue Constance and single-handedly fights off several of Richelieu's men. D'Artagnan is badly beaten in the fight but he quickly forgets his injuries when Constance gives him a small kiss. He then professes his love for Constance and they seal their love with a real kiss. Later, D'Artagnan follows Constance to her secret rendezvous with the English Duke of Buckingham. In a fit of jealousy, D'Artagnan challenges the duke to a duel, but the duel is called off when D'Artagnan learns that the furtive meeting has been arranged to deliver the duke to Queen Anne, with whom he is engaged in a secret affair. Before the duke returns to England, Anne gives him a keepsake of twelve large, perfectly matched diamond studs, which the king has given her as a gift. When Richelieu learns from one of his agents that Anne gave the studs to the duke, he sends an accomplice, the beautiful Lady de Winter, to England to steal two of the diamonds so that he can expose the affair and force the king to consent to a war with England. Determined to prevent the affair from being exposed, Constance sends D'Artagnan to England to get the studs before Richelieu finds them. D'Artagnan promises to return with the jewels before the banquet at which the queen is expected to be wearing all her diamonds. D'Artagnan enlists the help of the other musketeers to get the jewels back, and as soon as they arrive in England they engage Richelieu's men in a battle. D'Artagnan arrives at the duke's estate too late, and discovers that two of the diamonds have already been stolen by Lady de Winter. While Lady de Winters set out to deliver the diamonds to Richelieu, the king's musketeers search for Athos, who, in a fit of depression over his past, has gotten violently drunk. Following several misadventures, the musketeers take exact duplicates of the missing gems, which have been made by the duke's jeweler, to France in time to deliver them to the Queen for the banquet. His scheme foiled, Richelieu exacts his revenge by arresting Constance and holding her captive. Richelieu refuses to release Constance until D'Artagnan consents to join forces with him as his lieutenant. When D'Artagnan refuses, Richelieu instructs Lady de Winter to use her persuasive charms to seduce him. The other musketeers later condemn D'Artagnan for consorting with Lady de Winter in order to continue his romance with Constance. D'Artagnan, however, insists that he is merely trying to win Lady de Winter's confidence to find out where Constance is being held. D'Artagnan devises an elaborate scheme to trick Lady de Winter into revealing Constance's whereabouts, but he later undermines his own scheme when he succumbs to Lady de Winter's charms and reveals his plan. Meanwhile, the queen's aides find and release Constance and deliver her to D'Artagnan's quarters. A hasty wedding is arranged between D'Artagnan and Constance before Constance is sent to Buckingham for safety. Soon after, the war between France and England breaks out, and Lady de Winter follows D'Artagnan to England, determined to exact revenge against him for tricking her. Warned of Lady de Winter's treachery, the Duke arrests her and places her under Constance's supervision. When D'Artagnan learns of the arrest, he races to the Duke's estate, hoping to prevent a tragedy. He arrives too late, however, as Lady de Winter has killed both Constance and the Duke. The musketeers eventually find Lady de Winter, who it is revealed, was once married to Athos, and after delivering her to the executioner, they fight off the last of Richelieu's men and bring the war to an end.