'Commodities' reveals a good deal about Porthos (Howard Charles) - the last of the Musketeers to have his backstory sketched in, we learn that he grew up on the streets, the child of a slave mother - and more still about Athos (Tom Burke) - the frosty one's exterior finally cracks, as a return trip home unveils some dark secrets.
The episode's rather frothy first quarter seems to promise something lighter than the previous two weeks, but from the moment Porthos is injured in battle, 'Commodities' grows progressively darker.
This is the first time the Musketeers have really felt vulnerable - not only is one of them wounded, but their cocky camaraderie takes a serious hit too, as the difficult task of transporting a slave trader to Paris threatens to tear our heroes apart.
'Commodities' is thankfully far from po-faced, however - guest star James Callis provides plenty of humour and is clearly having a ball as the aforementioned trader Bonnaire, the Lord Flashheart of 17th century France.
The balance between comedy and drama only tips the wrong way at the very end - coming after a passionate and thoughtful debate about slavery, Bonnaire's return to slave-trading and his ultimate fate is played chiefly for laughs, which feels like a misjudgement in tone.
Elsewhere, one could still find fault in The Musketeers for its Boy's Own ethos - in Bonnaire's no-nonsense wife Maria, the show finally has a strong and vibrant female character to be proud of.
Naturally then she's killed off after two scenes - thank goodness at least for Maimie McCoy's Milady, whose on-screen interactions with Athos and the Cardinal (Peter Capaldi) are always a delight to witness.
Some flaws are yet to be ironed out, but there is at least a real feeling of growth in week three of The Musketeers - 'Commodities' starts out as a mere romp yet, in delving deeper into characters' backstories and engaging in social issues, it turns out to be the most thoughtful Musketeers episode yet.