D'Artagnan (Michael York) and his father (Joss Ackland) engage in an exhausting sparring duel. The father shows his son a signature move, which ends with the point of the sword at his son's chest, and advises to use it in a fight only as a last resort. He gives the boy 15 crowns and a letter to the commander of the King's musketeers, then presents him a sword he and his own father used in the King's service. The young Gascon leaves home on a huge yellow workhorse with his few belongings and a salve his mother advises him to rub into any wounds, and he strikes out for Paris to take his place as a musketeer.As D'Artagnan arrives in Paris he comes upon Rochefort (Christopher Lee), a tall man with a patch over one eye, who makes an offhanded remark to some companions about the newcomer's horse. Anxious to fight at any opportunity, as his father suggested, the young Gascon challenges Rochefort, only to have his sword struck in two. He is knocked out as his new enemy goes to meet an approaching carriage. Rochefort greets the passenger inside--Milady DeWinter (Faye Dunaway)-- and tells her she must go to England to report on the Duke of Buckingham (Simon Ward). The pair are spies for Cardinal Richelieu (Charlton Heston). D'Artagnan again tries to pick a fight but ends up falling in the mud.Finding the musketeers' headquarters, D'Artagnan awaits an appointment with their commander M. Treville (Georges Wilson). As he makes his way through the indoor training ground, he sees Porthos (Frank Finlay) having his portrait done while Aramis (Richard Chamberlain) watches; outside Treville's office, Athos (Oliver Reed) has a shoulder wound attended to. Treville reads D'Artagnan's letter and asks to see the familiar sword, which D'Artagnan sheepishly produces; the commander knows the weapon, realizes what has happened and lets him borrow a new one. Unfortunately there is no place in the musketeers for him, Treville says, until he has established his reputation on campaign or in killing, but a place in the guard might be suited to him in the meantime. Suddenly D'Artagnan spots Rochefort outside and leaps from the window. . .only to be hoisted back up as he lands on a scaffold. Excusing himself to Treville, he hastily runs from the office and collides with Athos and his wounded shoulder. The musketeer says D'Artagnan needs a lesson in manners, and they agree to duel at the Carmelite Convent at 12:00. Hurrying downstairs to catch up with Rochefort, the Gascon knocks Porthos down, causing a wardrobe malfunction which embarrasses him, so they agree to duel at the Luxembourg at 1:00. Once outside, D'Artagnan mistakenly compromises a lady trying to get Aramis' attention, so the musketeer and intended priest decides to have the young man meet him at 2:00 to settle the matter with swords.At the convent, it turns out that D'Artagnan's other two appointments are with Athos' dueling seconds. Before the duel can begin, however, six of the Cardinal's guards appear and prepare to charge at the musketeers for dueling, which is against the law. D'Artagnan joins the outnumbered threesome, noting that he may not have a tunic, but has the heart of a musketeer. Together they make short order of the guards, and Athos has an opportunity to see that the Gascon is a good swordsman. The remaining two guards surrender, but not before Porthos finds that his hat has been cleaved in two and lifts twenty pistoles from a downed guard to pay the damages (and twenty more to teach them manners). Later Athos explains that the musketeer motto is "one for all, all for one," so each of them gets ten pistoles from the booty, which Athos suggests D'Artagnan use to get lodgings and a servant. He chooses Planchet (Roy Kinnear) for a servant, and takes dingy lodgings at two francs a day with the elderly M. Bonacieux (Spike Milligan) and his wife Constance (Raquel Welch), a clumsy but beautiful younger woman and the dressmaker to Queen Anne of Austria (Geraldine Chaplin).Porthos bets and loses all of their money, so the group enter a tavern; Porthos makes it a point to annoy Aramis and they start to duel, but when D'Artagnan tries to quell the swordplay, Athos pulls him back. The men are creating a distraction while they rustle some much-needed food and drink. D'Artagnan and Planchet soon get into the spirit, passing the pilfered food to Athos, who hides it under his cloak. When they depart, Athos throws a purse to the barkeep, which he secretly filled with salt, they make their escape and fill their bellies with the hard-won food.At the palace, the Cardinal arrives, and locals are paid to wave flags and cheer his coming, as he is despised by many for manipulating the power of the King (Jean-Pierre Cassel) to his own advantage. The King discusses the musketeers' fight with Richelieu's "ruffians," and is worried because the Duke of Buckingham has arrived in France and covets his wife Anne, but Richelieu assures him that she is faithful; in secret he has a plot he is about to set in motion.At night Rochefort and his men break into M. Bonacieux's home while he is in bed with Constance, and prepare to take the couple to the Bastille, but Constance fumbles her way free and returns home and into D'Artagnan's arms. Later she leaves, saying she has to stop somewhere before she continues to the palace and the Queen, and makes him promise not to follow her, but he worries about her safety and trails after her from a distance to discover her meeting with another man. He challenges the interloper, then finds out it is the Duke of Buckingham himself. He helps escort the Duke and Constance to the palace laundry where Buckingham meets with Anne and begs her for a token he can hold until they can be together for good. She gives him a necklace containing twelve diamond studs. Suddenly guards appear and the Duke is outnumbered and in danger of being discovered, but D'Artagnan assists and Planchet brings the other musketeers as well, and they defeat the guards while the Duke makes his escape.At the Bastille, the Cardinal meets with Bonacieux and accuses him of treachery, then upbraids Rochefort for failing to hold onto Constance; Rochefort tells the Cardinal that Anne has given the diamond studs to Buckingham. The Cardinal releases Bonacieux, befriending him with a gift of 100 pistoles to get him to spy on his own wife. At an outing, the Cardinal mentions to the King that he wants to see the queen dance, and they plan a ball in her honor in two weeks' time. Richelieu suggests to the King that she should wear the diamond studs, and he approaches Anne with the request. The Cardinal later gives him a box with two diamond studs inside, and prompts him to count the studs when Anne wears the necklace and question her if they are all not there. He has sent Milady DeWinter to England and, in a tryst with Buckingham while he is wearing the token of Anne's affection, Milady has stolen the two studs the Cardinal has given the King. For the Queen to appear without them will cause her dishonor and may well cost Anne her life.Desperate to get the studs back, the Queen writes a letter to Buckingham and gives it to Constance, who is prepared to entrust it to her husband, but when he mentions that the Cardinal "took my hand and called me friend," she refuses to reveal more, so Bonacieux goes out to fetch Rochefort. D'Artagnan has been listening unseen, and volunteers to take the letter as he loves her and knows how to get to England. He locates the purse of 100 pistoles to use for the trip, but the pair are forced to quickly hide in a wardrobe when Bonacieux arrives with Rochefort and his men; after no trace of the pair is found, they leave. Constance faints and the wardrobe crashes to the ground. D'Artagnan breaks out of the wooden wardrobe, angered by the continued interference of the "inconvenient" Rochefort. Athos later tells the Gascon to avoid the man as he is a dangerous swordsman for the Cardinal. The trio of musketeers agree to help D'Artagnan get the letter to Buckingham and ride out with the agreement that if one falls, the others continue on their way.At a tavern Porthos gets into an argument with a man and, as they duel it out, the others ride on. The man slices into the musketeer's outfit, drawing blood, and Porthos drops to the ground. Later Athos and Aramis are set upon by thieves; Aramis is held at gunpoint, and D'Artagnan and Planchet watch from a distance, shocked, as Athos is cut at the throat in a duel. They continue to the docks to board a ship for England, but have no pass. Rochefort has a signed pass to board, but needs the governor to countersign, so he takes the road to town. On Rochefort's return, D'Artagnan accosts him, and when they recognize each other they fight a rough duel in the dark. Rochefort is wounded by D'Artagnan but not killed, and the Gascon suffers scraped ribs. Planchet nabs the pass, and when the harbor master questions why there are two people with one pass, D'Artagnan notes that he is one person, and Planchet is a servant.An exhausted D'Artagnan arrives in England, having taken six days. Buckingham is ready to hand over the studs but suddenly realizes that two studs are missing and figures out that Milady took them. His jeweler O'Reilly (Frank Finlay) is able to perfectly reproduce the two studs, and Buckingham hands the packet over to D'Artagnan, asking how he can repay him. D'Artagnan notes that they are enemies, but Buckingham offers a handshake and a sword which he accepts. With Planchet at his heels, they manage to get to Paris before the ball starts. At the familiar location of one of his companions' duels, D'Artagnan jumps upon a fresh horse, cutting a rope attached to the pommel, not knowing that two of his fellow musketeers are using the horse to pull Athos out of a well he has fallen into. The musketeers are wounded but recuperating. They realize from the accident with the horse that D'Artagnan has returned successfully and head out to join him at the ball.The Queen arrives on the arm of the King, but she is wearing a pearl necklace and he demands she wear the diamond studs; stalling for time, she says she will send to the palace for them and departs to wait with her dressmaker in hope of being saved. When Constance looks out a balcony window, she upsets a pot of flowers and D'Artagnan immediately knows from the accident where to go; he creates a distraction with musicians and prematurely set-off fireworks, but is delayed in getting the studs to Constance when the guards appear. The musketeers soon arrive and with the help of Planchet, who infiltrated the ball in a polar bear costume, they manage to subdue the guards. D'Artagnan throws the packet with the studs up to Constance, but they are thrown into the wrong window. Milady picks it up, but Constance bursts in and the two women fight until D'Artagnan swings in to assist. Constance is able to nab the valuable packet.The Queen reappears at the ball and reveals the studs at her neck. After the King tries unsuccessfully to count them while dancing with his wife, he stops the music and reveals the box containing the two studs which the Cardinal had given him; she delightedly says she now has fourteen studs. Backpedaling, the Cardinal says he wanted to give the additional studs to her but could not bring himself to do so directly. Milady informs the Cardinal that D'Artagnan thwarted the plot, but the Cardinal says the game continues.At an elaborate ceremony, Treville announces that D'Artagnan has become a musketeer; his comrades put the tunic over his shoulders while the commander provides his musket. The Queen greets him and, when she offers her hand, she discreetly gives him a ring as thanks for saving her honor. Milady flirtatiously passes by the new musketeer as D'Artagnan, joined by Constance and his friends, head to new adventures, and they become the Four Musketeers.