The Four Musketeers

During the Anglo-French War (1627â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 Bonancieux, dressmaker to the Queen of France. The evil Milady de Winter, who wants revenge on junior musketeer d'Artagnan, seduces him to keep him occupied. He soon discovers her true nature, however, and also that she was once married to his fellow musketeer Athos, who had supposedly killed her after discovering that she was a branded criminal.
The trio of musketeers â Athos, Porthos, and Aramis â rescue Constance from imprisonment in Rochefort's abode of Saint Cloud and take her to safety in the convent of Armentieres. De Winter sends d'Artagnan poisoned wine and a note intended to trick him into thinking that the trio have been imprisoned for drunkenness. On his way to bail them out, d'Artagnan is attacked by Rochefort and his men. The trio join the fight, and Rochefort flees. One of his men is captured and tortured for information, revealing that Richelieu is going to the Dovecote Inn near La Rochelle, but then drinks the poisoned wine and dies, revealing de Winter's trap. The trio then proceed to the inn where they spy on Richelieu. The Cardinal orders de Winter to threaten the Duke of Buckingham with exposure of his affair with the Queen, to discourage him from sending a relief force to aid the rebels; she is to kill the Duke if he does not comply. In return, de Winter asks for a warrant, so she can kill d'Artagnan and Constance. Richelieu reluctantly signs one, wording it in a way that leaves no evidence against himself.
After revealing himself to de Winter, Athos takes the death warrant from her and later tells d'Artagnan of the plot. D'Artagnan sends his servant Planchet to warn the Duke. In England, de Winter asks Buckingham not to help the rebels, but he refuses. De Winter tries to assassinate him, but she is captured. Buckingham has his servant Felton lock her away in the Tower of London, but she seduces Felton and convinces him that Buckingham is his enemy. Felton helps her to escape and return to France, then murders Buckingham before Planchet can warn him. Soon after, La Rochelle surrenders.
Rochefort and de Winter are still intent on killing d'Artagnan and Constance. With a force of guards, they occupy the convent at Armentieres and battle all four musketeers when they arrive. While Rochefort and his men hold the musketeers at bay, de Winter strangles Constance. 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. Afterward, they are arrested by the Cardinal's guards.
Richelieu charges d'Artagnan with murder for killing a valuable servant of the State, but d'Artagnan shows him the signed death warrant which, due to its ambiguous phrasing, appears to authorize d'Artagnan's actions. Utterly defeated and quite impressed at d'Artagnan's achievement, the Cardinal offers him a commission for either him or one of his three friends to become an officer. Athos, Porthos, and Aramis all reject it, and d'Artagnan is promoted to Lieutenant of the Musketeers.

Question: Who warns the Duke?
Answer: Planchet