At the beginning, she was The Impossible Girl who appeared throughout the Doctor’s lives, splintered into multiple incarnations to save him a dozen times over.

They travelled together as flirty friends in the days when the Doctor was fresh-faced, floppy-haired and full of sparkling youthful exuberance and cheeky good humour. That was then.

Now, Clara Oswin Oswald is an (extra)ordinary human being with a job, a boyfriend and a life, who still travels from time to time with a new version of the Doctor: a hyperactive, morally-ambiguous, craggy-faced old git with a tongue almost as sharp as his taste in jackets. Their relationship, as Ozzy Oswin Osborne might say, is going through changes.

The Doctor lies. His master’s degree in misdirection from the University of Bullshit has been put to good use for a long time. Clara, having stomached it for too long, is sick of being misled – and of being patronised, being used, being put at risk … basically suffering the companion’s lot since Ian Chesterton and Barbara Wright met a man in a junkyard in 1963. If her fury from ‘Kill the Moon’ has faded, Clara’s desire to stop travelling in the TARDIS lingers throughout their jaunt on the Orient Express.

It’s impossible to choose endings; endings choose you. The Doctor and Clara have arranged a final hurrah to close the door on their peripatetic career, but trying to schedule a breakup is an exercise in futility. Certainly, the upheaval of a murder mystery on a luxury space train with an emotionally-vacillating computer is hardly the place to say goodbye to the good times. It’s no surprise that they’re still together by the end of the episode – but what of the future?

Danny Pink has encouraged Clara to stay with the Doctor. Her enthusiasm seems renewed by the time their intergalactic rail trip comes to an end. Yet the Time Lord has not noticeably mellowed – his wild-eyed enthusiasm is as brusque as ever – and Clara is hardly going to passively-aggressively demur, Peri-like, to his arrogant superiority. How long will it be before the problem blows up again?

Pretty soon, most likely. There’s more friction between these two characters than pretty much any previous Doctor/companion team. And why? Because the Doctor is in love with her and she doesn’t feel the same way.

Heresy? Treasonable cack from the depths of slash fiction? Not entirely. The Doctor has fallen for a companion in the past – the grand passion with Rose was a central tenet of the Tennant era.

More recently, look at the Doctor’s impassioned speech at the end of ‘Deep Breath’ … and his delight during ‘The Caretaker’ when he mistakenly assumed that Clara was going out with someone who resembled his previous self … and his broken dismay at her furious departure in ‘Kill the Moon’ … and his delighted relief when she decides to stick around for more adventures in ‘Mummy on the Orient Express’. Despite his caustic bluster, the Twelfth Doctor clearly has feelings for Clara – and he’s certainly never demonstrated quite such hostility towards the boyfriends of his companions in the past.

Rory Williams, he liked. Mickey Smith, he tolerated. Even David Campbell, the man for whom his granddaughter Susan deserted him, he grudgingly respected – although he probably wouldn’t have if David had possessed the surname bestowed upon him in the novelisation of ‘The Dalek Invasion of Earth’ (Cameron).

The Doctor’s antipathy towards Danny Pink, on the other hand, is deep and complex. He hates soldiers; he hates that Danny is a former soldier; and he hates that Danny recognises the fact that the Doctor is a soldier himself. But could his constant curtness be cover for something more basic? The fact that Mr Pink is seeing the girl who has stolen the Time Lord’s hearts?

Implausible, perhaps, but this current season has revealed the moon to be an egg, Robin Hood to be real and that leaking radiation to Daleks is like LSD to hippies.

Is a lovelorn Time Lord really any less likely? As Clara herself can attest, nothing is impossible. Not even her.

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What do you think the future holds for Clara and the Doctor? Let us know below…