But back to Lady Sybil, who was approaching labour and suffering swollen ankles and headaches. There were worse complications in the drawing room, thanks to Lord Grantham's decision to bypass Clarkson, the reassuringly Scottish local GP, and bring in galloping upper-crust obstetrician Sir Philip Tapsell.
Tapsell announced himself as a bit of a lunk straight away over dinner, boasting that he'd wangled three boys out of a nervous duchess only recently, and condescendingly dismissing the possible input of Dr Clarkson, albeit while allowing him to attend the birth.
Once everyone was on the outside of pudding, Matthew waylaid Tapsell on his way to examine Sybil. Earlier, Matthew and Mary had shared another cryptic conversation about parenthood, after last week when she seemed awkward about his reference to a child's nursery. As Matthew outlined his plans to save Downton from Lord Grantham's mis-management by asking their decrepit old farmer to rethink his "derelict buildings with no visible livestock" plan, again there was a hint that all might not be well between the four posts of their bed.
It turned out Matthew was worried that his series two spinal injury, which Clarkson had warned might make him for ever unable to rise for the national anthem, might not have been miraculously healed after all. He blushingly confirmed he was flying at full mast, but he'd been married to Mary for a few months now – why was there still nothing in the oven? Sir Philip said anxiety wouldn't help and told Matthew not to worry.
Telling people not to worry because everything would be dandy was the extent of Sir Philip's medical acumen. Dr Clarkson observed that Lady Sybil seemed muddled and was displaying eclampsia symptoms; Sir Philip waved him away. Dr Clarkson demanded to test her urine; Sir Philip bridled. Dr Clarkson suggested that if they didn't cart Sybil down to the hospital for a C-section pronto, she and the baby might die. Sir Philip downright pooh-poohed it.
The arguments went on, but then suddenly the tension and drama dissipated as Sybil gave birth to a healthy girl off-screen. There have been a few plot lines resolved rather too neatly this series, but this was the most anticlimactic yet.
Or so we thought, until Sybil announced that she was going to sleep now, and hurriedly imparted her wishes to Mary re Tom's plan to work as a chauffeur in Liverpool and christen the baby as a Catholic. Uh-oh...
Genuinely harrowing and brilliantly acted scenes followed as Sybil started seizing in the middle of the night and then asphyxiated as her family and a rather sheepish Sir Philip looked on. All those lightly comical moments elsewhere had been a diversion to disguise the coming death of a major character, albeit one who hadn't been in this series much because she'd already started doing films and other TV programmes.
In the aftermath, Lady Grantham blamed her husband for siding with Sir Philip, Edith asked Mary if maybe they'd be brought closer by their sister's demise ("I doubt it," said Mary, further adding to the material for Edith's bumper first newspaper column), and Matthew put his foot in it massively by being caught discussing the estate's accounts with Downton's solicitor before Sybil had even been carted off.
The tragedy was the sort of extreme situation where experienced old hands take control: sure enough, in came Maggie Smith. The Dowager Countess arrived at the house the next day, veiled in black, greeted by an ashen Carson. "We've seen some troubles, you and I. Nothing worse than this," she said, laying her hands on his before setting off shakily across the lobby, filmed in isolation, a lonely old woman getting ready to bury her grand-daughter.
Smith paused halfway to let her face flicker with grief – a moment to remind us that despite all the soapy giggles, Downton Abbey can still sometimes deliver a mighty emotional punch.
>> Series three, episode four: Branson breaks the law, the servants go sex-mad