The fourth episode of The Musketeers is full of political intrigue, and spies, and potential treaties how sometimes it is better to hide something from somebody in order to get your own way.
The main plot is that a treaty is to be signed so that France and the Kingdom of Savoy will be closely linked together which will give the French more power and will, best of all, piss of the Spanish.
When the Duke of Savoy arrives there is a botched assassination attempt on his life, and Aramis discovers that the culprit was a former musketeer and good friend of his who he had last seen when there was a massacre of Musketeers some years before, who tells Aramis that it was Savoy who was responsible for the massacre than it was Treville who betrayed the Musketeers.
Not wanting to believe that Treville would betray his own men Aramis and the others try to piece together what happened and why it happened and came upon the truth that sometimes things are done for unpalatable reasons for the greater good and that was one of those times and like it or not it was the best thing to do at the time.
Cardinal Richelieu has plenty to do in this episode as he was a major part of the treaty, and the use of spies, in order to make sure that the treaty was signed at all costs, for the benefit of France, and also for the benefit of himself in some way, because I doubt he would do it just for the benefit of others, he doesn’t seem to be that sort of character.
Treville is shown in this episode as ultimately just a soldier following orders, which is why he did what he did, no matter what the cost was, as that was what he was supposed to do, and that is what a solider is always supposed to do, so he wasn’t technically responsible even if he felt like he was. Such is the soldier’s lots, I would imagine.
D’Artagnan is seen getting closer and closer to Constance in this episode (most notably in the scene where she asks D’Artagnan to teach her how to shoot, and he gets up close and personal with her whilst doing so), and she isn’t exactly discouraging his flirting either, the dirty little hussy!