In year one, Porthos was the most routinely neglected of the Musketeers, afforded a single centric episode ('The Homecoming') rather than sustained development throughout the series.
But the burly brawler's quest for family has underpinned his journey this year - just one example of how The Musketeers has grown into a more layered, balanced and sophisticated series this year, while never losing its inherently frivolous charm.
On death's door last week, Treville finally revealed the identity of Porthos's Pops and in 'The Prodigal Father', he and the audience at last meet Belgard (played by the terrific Liam Cunningham, always guaranteed to deliver a charismatic and engaging performance).
Once captain of the royal bodyguard, Belgard failed to prevent the assassination of Louis' father King Henry and has lived in seclusion ever since. His sentence, so he says, was secured by Treville - who seemingly gave false evidence against his comrade.
By his own admission, Treville committed a "cruel" and "unforgivable" sin in his treatment of Porthos and his mother - and elsewhere this week, d'Artagnan comes off as rather selfish and impatient in the wake of Bonacieux's demise.
While a widowed Constance is wracked with guilt, d'Artagnan - who hesitated before attempting to save his love rival's life - certainly doesn't hesitate in celebrating his demise. A family adventure series needn't explore such moral shades of grey, but The Musketeers is all the richer for doing so.
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Aramis, meanwhile, redeems himself a little after weeks of wanton womanising - his unwise affair with Marguerite behind him, it's rather fitting that he uses his reputation as a ruse to cover for some outright heroism, putting Porthos' well-being ahead of his own 'desires'.
Because there's little drama in a happy family reunion, when Belgard warmly embraces Porthos as the son he's always wanted ("a champion"), it seems inevitable that he'll be unmasked as a menace and 'The Prodigal Father' playfully toys with our expectations on that front.
Is Belgard a villain or is Treville allowing old prejudice to cloud his judgement? The truth is revealed, though not before Porthos - desperate to connect with his deceitful father - is turned against his comrades, even briefly resigning his commission.
We know this resentment won't stick - forced to choose between his two 'fathers', there's never any doubt as to which way he'll turn - but it's heartening to see something more asked of Howard Charles and he acquits himself admirably with this unusually emotional material.
Beyond the perils of Porthos's parentage, 'The Prodigal Father' somewhat stalls on this series' subplots - Milady knows the truth about Rochefort, but rather than expose him outright, she decides to poke and provoke her nemesis.
He even proposes an alliance to expose Aramis's affair with Queen Anne - it's a rather sudden and improbable twist given that Rochefort and Milady have been at each other's throats (once literally) ever since their first encounter and said pact comes to nothing.
It feels a little like stalling until the admittedly spectacular final scenes, in which Marc Warren's slippery super-villain proves to be his own worst enemy - exposing his lust for both power and for the queen as he attempts to force himself on Anne, taking a hair pin to the eye (ouch!) as she fights him off.
On the surface, much of the 'The Prodigal Father' is Porthos' story and, like 'The Homecoming', it initially feels rather standalone - but dig deeper and it does delve into some of the themes recurring throughout this series - chiefly the subjugation of the weak and the poor by the bullying aristocracy.
It's another strong example of what has become this show's standard; an adventure story with plenty of intrigue, wit and emotion that also offers up some gentle exploration of social issues. The series arc may take a backseat for the majority, but those final scenes are a tantalisingly dark teaser for what look set to be an explosive final two episodes.
