Oh, Downton Abbey – how could you? You go through an entire world war and barely kill off a soul, and now, in time of peace, you sacrifice the youngest and demurest of them all!

Spoilerphobes who have managed to avoid the outpouring of grief on Twitter – look away now! For, in a rare display of writerly kahunas, Julian Fellowes has only gone and killed off Lady Sybil! Foolish enough to entangle herself with the working classes, the weight of aristocratic tradition has visited itself upon Sybil’s beautiful head, and she has been punished as all such women shall be with the curse of her gender: death through childbirth.

Toxaemia, eclampsia – call it what you will – it’s the classic Cordelia Gambit of sacrifice of the innocent. Of the one true daughter everyone liked. And now the Holy Trinity of Crawley daughters has been reduced to an uneasy two. There’s a tentative rapprochement forged between Mary and Sybil. But that initial Pride and Prejudice-style set-up of the adventures and misadventures of the eligible daughters of the house has now been obliterated.

For Sybil, it is perhaps as well. Reduced last series to popping in and out of the garage every six months, to resume an ongoing almost-conversation with the chauffeur, she has been even more sidelined this year. Her future seemed assured: a few storylines as a put-upon buffer between the forces of Protestantism and Catholicism – the bestower of sincere and saintly smiles to acknowledge how very difficult this business of Home Rule is – then, finally, a turn as a Diana Mosley-style supporter of Fascism when Downton finally hit the 1930s.

But no. For once, Fellowes has dared to throw in a narrative curve-ball which will have implications for the rest of the series’ life. No sudden plot resolutions here – only resentment of Robert from Lady Cora, and a chance for ham-fisted doctor, Tim Pigott-Smith, to reclaim his title as biggest onscreen Imperialist baddie, thirty years after The Jewel in the Crown.

And there we were, thinking that the biggest plot strand this episode would be Thomas’s attempted seduction of the new footman: all hands pressing urgently on clock faces and the accompanying, sotto voce, enquiry, ‘Do you feel a slight increase in the resistance?’

Downton being Downton, we don’t entirely trust Fellowes to follow this most devastating event without a little bonkersness. That Matthew and Mary should adopt the baby Sybil seems too obvious – though Downton has never avoided obviousness before. On the nation’s settees, jaws, having now been picked up from the floor, are busily engaged in the business of speculating on Branson and the baby Sybil’s future. Will Branson have his Falling Down moment and blow the whole complacent lot of them up?

Still, there were signs of a commitment to a less melodramatic storytelling approach in one moment: a shot of the Dowager Duchess of Grantham, in mourning dress, looking suddenly frail and small and old on arrival at the Abbey – a stately galleon briefly capsized before resuming the business of dispensing no-nonsense advice. It was a moment of authentic human detail amid so much sentimentality. An acknowledgement that, however contrived the storytelling, when Downton gets it right – when it rocks our Sunday nights and leaves us with a heady hangover for work on Monday – it does so foremost through the people.

Aired at 9pm on Sunday 14th October 2012 on ITV1.

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