Given how multi-faceted the storylines on Mad Men can be, it wouldn't have been at all surprising to see the show drift away from the Don / Joan relationship in order to explore other characters, so it's a delight to see this dynamic given more screen-time in 'The Other Woman'.
Last episode, Don rallied his troops as the agency prepared to pitch for Jaguar and this week the campaign - led by the ever-neurotic Ginsberg (Ben Feldman) - is in full swing. But when Pete (Vincent Kartheiser) and Ken Campbell (Aaron Staton) meet with Herb Rennet, head of the Dealers Association, he makes a rather unorthodox request - he'll back their bid, but only in exchange for a night with Joan.
Joan's always been the number one sex symbol on Mad Men and while the character's certainly used her sex appeal to get what she wants in the past, 'The Other Woman' marks the first time that the men around her seriously consider exploiting Joan's charms for their own gain.
The scenes in which the agency partners - crucially minus Don - plot to indulge in some "dirty business" to secure Jaguar's business is deeply seedy and unpleasant, the scene in which Joan finally hands herself over to the lecherous Herb (Gary Basaraba) even more so.
Yes, despite a horrific early pitch from Pete and a somewhat unnerving follow-up from lovestruck Lane (Jared Harris), Joan opts to spend the night with a complete stranger for a stake in Sterling-Cooper-Draper-Pryce. In many ways, Mad Men has always been the story of its women - the rise of Peggy from unhappy secretary to successful copyrighter has been heartening to watch, for example.
But we're unsure how we feel about this recent development. On the one hand, Joan has become more powerful than ever - truly the equal of her male counterparts. But on the other, she has utterly debased herself and allowed herself to be manipulated by those same men.
We'd tend to side with Don, who does his best to prevent Joan from sealing the deal. The way in which this plot unfolds on-screen is simply brilliant - first we witness Don imploring Joan not to go through with it, then we see her horrid encounter with Herb...
Only later do we realise that the latter precedes the former. Don was too late to stop Joan and it's implied that if she'd only known he was on her side, she wouldn't have done what she did - a tragic and heart-sinking moment. The moment when a deflated Don discovers that Joan has apparently gone against his wishes is no less moving.
Elsewhere this week, Peggy (Elisabeth Moss) quits her job, leaving her friends and colleagues behind for good. Yes, you read that right. It's a sign of how packed with incident this episode is that this game-changing sequence of events - altering something that's been fundamental to this show since its inception - is pretty much relegated to a sub-plot.
But while the Joan plot may take precedence, we're not suggesting that Peggy doesn't get her due. Tired of toiling away without recognition and suffering one humiliation too many at Don's hands, she takes a job at a rival agency and the scene in which Peggy finally musters the courage to hand in her notice is utterly superb.
Bidding farewell to her friend and mentor, Peggy extends her palm for a handshake, only for Don to tenderly kiss it instead - it's a wonderful moment, expertly played by Moss and Hamm. Peggy is bold but sad, Don is proud but bitter - it's easily the two actors' best work together since season four's 'The Suitcase'.
Back in this season's third instalment 'Tea Leaves', an exasperated Roger moaned, "When are things going to go back to normal?" Well, now we know the answer - they're not. Peggy is gone, apparently for good, and not only has Joan been made a partner in the firm, but she's also made a moral choice that there's no way back from.
If the final two episodes of season five are anyway near as dramatic and surprising as 'The Other Woman' - and we've got a sneaking suspicion that they might be - then we're in for a hell of a ride.
Mad Men airs on Tuesday nights at 9pm on Sky Atlantic. It airs in the US on Sundays on AMC.
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