'Through a Glass Darkly' opens on an atmospheric teaser unveiling the astronomer Marmion's camera obscura - it's a brief but really rather beautiful sequence, tipping the audience from the off that this a rather special Musketeers outing - something a little different to your standard.
Each week, The Musketeers delivers reliably rousing entertainment, but 'reliable' can be a dirty word in drama and there's always the risk of growing stale.
This episode though boasts a somewhat different flavour to the norm, as our heroes venture out of everyday Paris into what feels - as much to the viewer as to the characters - like a magical world.
But the nature of subsequent plot twists mean it's a magic tempered with darkness, with Marmion (the terrific Leo Gregory) plotting a terrible fate for King Louis (Ryan Gage) and his subjects.
Gambling his own fate and that of others on the toss of a coin - like some sort of 17th century Harvey Dent - Marmion's backstory is darker territory than The Musketeers would normally dare to tread in, with his infant children starving to death as a result of what he considers to be the King's "indifference".
Dark, yes, but it also makes him a more nuanced villain than those the Musketeers often come up against, while - as in this year's 'An Ordinary Man' - exposing Louis to serious peril allows Ryan Gage to delve a little deeper, particularly as the King's subjected to psychological torment by the sadistic Marmion.
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Marmion harbours a burning dislike of the aristocracy and in particular bears a grudge against the Royals - "Some of us have the dice loaded in our favour by wealth and privilege," he scoffs.
This gulf between the rich and the poor that so infuriates him has become a recurring theme on The Musketeers this year - as it's grown more confident, the show hasn't shied away from giving social issues a little gentle needling alongside all the derring-do.
"I wasn't [a monster] once, but this world made me one," Marmion argues - and while The Musketeers will always have its heroes and its villains, it's heartening to see the show being less afraid to branch out from purely black-and-white moral territory.
Not that Marmion is any less of a menace for it - he means business, tossing Aramis from a window for daring to speak against him. Fortune favours the handsome though and the Musketeer lives - and Porthos's look of respect when he learns his friend has survived is wonderful.
Aramis's subsequent efforts to save the Queen and Marguerite are complicated by his recent womanising, with both women a little overly eager to 'express their gratitude' - it's unclear if Aramis will ever pay any real price for his actions, but you suspect Marguerite might throw him out of another window were to she to confirm her suspicions regarding his feelings for Anne.
Along with Aramis's survival, another element that Marmion hasn't banked on is the wiley Milady de Wynter - though she risks her life to save the others, her motives remain enjoyably ambiguous.
We know by now she's self-serving, but is she entirely selfish? Does she act out of empathy or simply to save her benefactor the King? She herself proclaims the latter - though her claims smack of emotional self-preservation.
In the event, Milady loses the King's support but regains Athos's respect - something which you suspect means a lot more to her, even if she won't admit it.
She's far from the only one forced to forge unexpected alliances in 'Through a Glass Darkly' either - though he remains utterly antagonistic in attitude, Rochefort unites with his Musketeer foes this week against a common enemy.
It's he who finally executes Marmion - his actions seeing him again hailed a hero - though while locked away with Porthos, Rochefort reveals the full extent of his dangerous delusions, as he insists that the Queen (whom he doesn't name) is in love with him.
Having rather more luck with a married woman is the impetuous D'Artagnan, who finally succeeds in winning over the more level-headed Constance, after declaring his love and almost giving up his life to save hers. It's a sweet end on which to end an otherwise rather bleak tale - even Trevile smiles!
The antics of an evil astronomer might not immediately sound like the strongest material for a stirring period adventure, but 'Through a Glass Darkly' is The Musketeers firing on all cylinders. Darker than the norm, yet somehow even more charming and exhilarating into the bargain, this is easily one of the show's finest episodes to date.
