Spanish General Tariq Alaman attacks Treville and demands an audience with King Louis.
Tariq demonstrates the power behind his explosive formula by blowing up a statue with only a few grains of powder. Louis is impressed and demands Tariq give him the formula. Tariq refuses to deliver unless the Musketeers rescue his kidnapped daughter, Samara.
Balthazar, a Spanish agent in Paris, is currently holding Samara hostage.
At the palace, Constance and Marguerite realize that the Dauphin is sick. A doctor is summoned, but Constance doesn't agree with his drastic methods.
Milady de Winter sneaks into the palace, but she is distressed to find the King treats her as just another subject. When she corners him again later, he rewards her persistence with a private dinner.
The Musketeers plan an exchange - the formula cipher for Tariq's daughter - but Aramis botches it because his mind is on his sick son. Porthos is captured in the scuffle.
Rochefort plans to use the Dauphin's illness to endear himself to the Queen. Meanwhile, when Constance hears the physician plans to use leeches on the baby, she steals the Dauphin from his cradle and takes him away from the palace.
At Louis' private dinner with Milady, she seduces him, and they spend the night together.
The King agrees to hand Tariq over to Perales as a gesture of good will, but the Musketeers follow their convoy to Balthazar's hideout. They storm the safe house, rescuing Samara and Porthos, but not Tariq or the cipher. In a last attempt to keep the cipher out of Spanish hands, Tarique blows the safe house up, with him and Balthazar inside.
When Anne realizes her son is gone, she commands Rochefort to search for him. He finds Constance and the baby in a laundry house. Louis orders Constance hanged for treason, but the physician informs them that the baby's lungs have cleared and he will survive. He credits Constance's "steam therapy" as the cure, and she is pardoned.