Similar to last week's instalment, 'The Good Soldier' uses a thriller plot to shed light on characters' backstories - both Treville and Santiago Cabrera's Aramis benefit this week, while Adrian Hodges's script also makes better use of Peter Capaldi's Cardinal than the past three episodes have managed.
The episode concerns a visit to France from Vincent Regan's brutish Duke of Savoy - both King Louis (Ryan Gage) and Richelieu are eager to have the Duke sign a treaty and lure Savoy away from their enemies in Spain.
But sins of the past soon threaten the alliance, as allegations emerge that the Duke may once have led a massacre of 20 musketeers, leaving Aramis and one other - Marsac - the only survivors.
'The Good Soldier' is bolstered by excellent performances from the regulars and a solid guest cast - there's more than a hint of Tom Hiddleston in JJ Feild's performance as damaged deserter Marsac, while a moustache-twirling Capaldi is clearly relishing Hodges's arch dialogue.
Credit to Hodges too for not having Treville's apparent involvement in the massacre turn out to be a total red herring. At first, you suspect 'The Good Soldier' will take the easy path and have the Captain be falsely accused, but revealing that he betrayed his men unwittingly and covered it up is far more murky and far more interesting.
If there's one flaw in this episode, it's that the Treville / Aramis / Marsac plot leaves the political goings-on at the palace rather in the shade. In particular, Regan's Duke comes off as a rather dull, one-note villain.
But in all other aspects, The Musketeers continues to impress in week four. One of the show's biggest issues - that its women are second-class characters - is even addressed here, as Tamla Kari's Constance pursues lessons in sword-fighting and gunmanship.
Let's hope these developments continue apace, because otherwise it's difficult to find fault in 'The Good Soldier' - a pacy thriller that's led by character rather than empty spectacle.