We begin with the funeral of Cardinal Richelieu, whose actor has sadly left the series to go play the infamous Twelfth Doctor.
Meanwhile, our musketeers just barely save the informant they're supposed to meet from being hanged for murder. Said informant is the nefarious Compte de Rochefort, and he uses the musketeers distraction to attempt another escape. They catch him and bring him to the King.
In the palace, Queen Anne has just given birth to the new Dauphin of France, a fact that has made King Louis very happy. He's still completely unaware that the child might not actually be his.
With the Cardinal dead, Louis offers his place on the council to Captain Treville, who declines. He is a solider, not a politician. The King is not exactly understanding, and he bitterly accepts Treville's rejection.
Rochefort's information is straight out of a Spanish prison, which he has apparently just escaped. The musketeers don't exactly believe his story, but they follow him into Spain anways on a risky mission to rescue General De Foix, a man who is still being held captive there. Their secondary mission is to kill him if they cannot rescue him, since he has sensitive military information that they cannot allow to fall into Spanish hands.
Constance and D'Artagnan share a moment, but she begs him to stay away from her so as not to anger her husband.
Aramis meets his son, and begins a rapport with the governess.
Rochefort meets up with the Spanish ambassador and reveals that he is a Spanish spy, released to infiltrate King Louis' inner circle by rescuing De Foix. The Musketeers are walking into a trap. Despite Rochefort's meddlings, the musketeers manage to rescue De Foix and his imprisoned sister, Lucy, but De Foix is shot in the escape.
Back at court, Rochefort receives all the praise, and a spot on the council.
Lucy and D'Artagnan share a kiss, which Constance witnesses.
Treville and De Foix reveal that they know who Porthos' father is: a man named Belgard.