During the Anglo-French War (1627–29), which involved suppression of the Protestant rebels of La Rochelle, Cardinal Richelieu continues the machinations he began in The Three Musketeers by ordering the Count de Rochefort to kidnap Constance Bonacieux, dressmaker to Queen Anne of France.

The evil Milady de Winter, who wants revenge on junior musketeer d'Artagnan, seduces him to keep him occupied.

The trio of musketeers — Athos, Porthos, and Aramis — rescue Constance from imprisonment in Rochefort's abode of Saint Cloud and take her to safety in the convent of Armentières.

De Winter sends d'Artagnan poisoned wine and a note intended to trick him into thinking that the trio have been imprisoned for drunkenness.

Athos captures de Winter; D'Artagnan duels Rochefort and apparently kills him with a lunge through the chest (though it is revealed in the sequel The Return of the Musketeers that he actually survived the wound).

The four musketeers sentence de Winter to death by beheading, and they hire an executioner to carry out the punishment.

During production on The Three Musketeers, the producers realized that the project was too lengthy to complete as intended — as a roadshow epic with intermission — and still achieve their announced release date.