(The scond of two companion Three Musketeers movies produced by Alexander Salkind. The movies were going to be one longer epic, but to make more money, the producers split them into two movies without telling the cast. A lawsuit ensued and now the "Salkind clause" exists so that a cast must be informed how many movies they are expected to be a part of.)
While the Four Musketeers fight in the war between the Catholics and the Protestants at La Rochelle, Milady de Winter (Faye Dunaway) conspires to have D'artagnan's (Michael York) lover Constance (Raquel Welch) kidnapped. She seduces D'artagnan, but when he finds out Milady is the disgraced ex-wife of Athos (Oliver Reed), he flees for his life.
The Four Musketeers rescue Constance and learn of a plot where Milady de Winter is given a carte blanche. from Cardinal Richileu to have the Duke of Buckingham, Constance and the Musketeers killed. Athos finds Milady and takes the carte blanche from her. She is captured and imprisoned before she can kill the Duke of Buckingham, but she seduces a Puritanical jailer Felton (Michael Gothard) who releases her and then travels to England to kill Buckingham. Felton is killed while slaying Buckingham and La Rochelle surrenders.
Constance is hiding at a convent. Milady sneaks in disguised as a nun and strangles Constance to death while Rochefort (Christopher Lee) and the Cardinal's guards fight the four Musketeers. The Musketeers defeat the guards, Athos captures Milady and D'artagnan kills Rochefort.
The Musketeers sentence Milady to beheading. After her execution, the Musketeers are arrested, but the carte blanche is worded very generally and it clears them of their actions. Impressed, Richelieu offers D'Artagnan and one of the Musketeers an officer's commission. Athos, Porthos (Frank Finlay) and Aramis (Richard Chamberlain) decline the offer, and d'Artagnan is promoted to Lieutenant.
Thanks Evil Ed!