Based on the first half of the Dumas novel, D'Artagnan joins three Musketeers in a special mission to stop a traitorous intrigue.
In 1625 D'Artagnan (Walter Abel) is sent off from Gascony by his father with his sword. Count de Rochefort (Ian Keith) ridicules D'Artagnan's horse and gives Milady de Winter (Margot Grahame) a secret treaty to take to the Duke of Buckingham. D'Artagnan tries to fight Rochefort but is knocked out. In Paris King Louis XIII (Miles Mander) objects to dueling. D'Artagnan shows his father's letter to Captain de Treville (Lumsden Hare). D'Artagnan quarrels with Athos (Paul Dukas), Porthos (Moroni Olsen), and Aramis (Onslow Stevens) and plans to duel each; but, arrested for dueling, together they fight six guards. The four sing and get D'Artagnan a room by threatening to arrest the landlord. D'Artagnan gets Planchet (John Qualen) as a servant. Rochefort gives Cardinal Richelieu (Nigel de Brulier) a message from de Winter that the Duke of Buckingham is coming. Constance (Heather Angel) sends away her guardian, and D'Artagnan sees her signal from his room. Queen Anne (Rosamond Pinchot) stops D'Artagnan from fighting the Duke of Buckingham (Ralph Forbes) and makes the Duke swear not to make war against France. D'Artagnan heard, and Constance has the Queen trust him. D'Artagnan escorts the Queen and Constance in the street. The landlord is arrested, talks to Rochefort, and is given money. Rochefort tells Richelieu that the Queen gave her diamonds to Buckingham.
Constance blames D'Artagnan and says the Queen must wear the diamonds. D'Artagnan asks his Captain for leave to get them. Athos, Porthos, and Aramis go with him. Athos gives Constance a ring and says where to meet. Rochefort orders them stopped; but they escape as the gate closes though Porthos loses his horse. The three are attacked on a bridge, but D'Artagnan and Athos get away. De Winter gets a message by pigeon from Rochefort to get Buckingham to sign. At an inn Athos and D'Artagnan are attacked, and D'Artagnan flees. At Calais D'Artagnan finds de Winter and dines with her to get a passport. She says she is in danger and shows him the diamonds. D'Artagnan offers to protect her. De Winter has D'Artagnan tied up in the coach and takes his letter from the Queen to Buckingham. At the tavern Athos sees de Winter write, but she knocks him out. Athos, Porthos, Aramis, and Planchet chase her coach and rescue D'Artagnan. Athos says that de Winter murdered his brothers and was his wife, exposing the brand on her shoulder. D'Artagnan finds the treasonable treaty of Rochefort. De Winter jumps off a bridge and drowns. D'Artagnan tells Constance to pose as de Winter to get in the palace. King Louis sees D'Artagnan dueling Rochefort, and the Queen proves Rochefort's treason. The Queen thanks the four, and the King makes D'Artagnan a musketeer.
Unable to keep "his hand out of dueling, his head out of politics, and his heart out of love," D'Artagnan (and the audience) thrive on his adventures in a still chivalrous age.