The Three Musketeers is a novel by Alexandre Dumas.
The poor d'Artagnan travels to Paris to join the Musketeers. He suffers misadventure and is challenged to a duel by each of three musketeers (Athos, Aramis and Porthos). Attacked by the Cardinal's guards, the four unite and escape.
D'Artagnan and his new love interest, Constance, help the French queen give a particular piece of jewelery to her paramour, the Duke of Buckingham. The Cardinal learns of this and coaxes the French king to hold a ball where the queen must wear the jewelery; its absence will reveal her infidelity. The four companions retrieve the jewelery from England.
The Cardinal kidnaps Constance who is later rescued by the queen. D'Artagnan meets Milady de Winter and discovers she is a felon, the ex-wife of Athos and the widow of Count de Winter. The Cardinal recruits Milady to kill Buckingham, also granting her a hand-written pardon for the future killing of d'Artagnan. Athos learns of this, takes the pardon but is unable to warn Buckingham. He sends word to Lord de Winter that Milady is arriving; Lord de Winter arrests her on suspicion of killing Count de Winter, his brother.
She seduces her guard and escapes to the monastery in France where the queen secreted Constance. Milady kills Constance. The four companions arrive and Athos identifies her as a multiple murderess. She is tried and beheaded.
On the road, d'Artagnan is arrested. Taken before the Cardinal, d'Artagnan relates recent events and reveals the Cardinal's pardon. Impressed, the Cardinal offers him a blank musketeer officer's commission. D'Artagnan's friends refuse the commission, each retiring to a new life, telling him to take it himself, and so he takes it and later on he becomes a well known lieutenant.
