What happened was this. Writer Stephen Greenhorne made a seemingly innocuous comment in an interview with Doctor Who Magazine, where he mentioned that the Doctor, as a character, never changes. Showrunner Russell T Davies read this and decided to challenge Greenhorne with an idea which would change the Doctor and see him develop as a character. His idea: to give him a daughter. And so here we are, with an episode designed to disrupt the Doctor’s cosy little world, and show us something new about him.

Looking back on it now, it’s a pleasing enough episode, if oddly structured, but it certainly hasn’t had that disruptive effect on the Doctor which Davies was aiming for. The addition of the Doctor’s daughter, Jenny (Georgia Moffett) may have proven a diverting development for 45 minutes or so, but at the end of the episode, she jets off into the night sky, never to be seen again. Did Davies plan for her to return, and be an ongoing fixture of the Doctor’s life – something which may well have changed the Doctor forever? It seems not – he brought back a panoply of companions for The Stolen Earth but not this game changing daughter he invented.

It’s in part because Jenny’s the Doctor’s offspring only in a sci-fi loophole kind of way. She’s created by some gene extrapolating whatsit designed to spit out new soldiers as fast as they’re killed. She’s comes with that get out clause; she’s only the Doctor’s daughter if we want to read it that way. Because of that caveat, and the fact that she’s never returned to the show, the whole idea feels gimmicky. A sensationalistic mid season gambit to combat the regular ratings sag around episode 6.

It’s one of a number of elements which gives The Doctor’s Daughter an air of not quite reaching its full potential. Consider also the fishy Hath who burble through mini tanks on rainbow coloured heads; a little too bizarre to sympathise with. Consider companion Martha Jones (Freema Agyeman) brought along for the ride but relegated to an irrelevant subplot. Consider the central conceit that the warring parties on Messaline – the Hath and the belligerent humans – have been fighting a pointless war for only a week… it’s a neat twist, but it adds um, what exactly to the story?

The irony is that it was the casting of Georgia Moffett which had the real impact. If not on the Doctor’s life, then certainly on actor David Tennant’s, as before long the two were a couple and now have a family together.  It’s impossible to watch those closing scenes of the Doctor nursing Jenny in his arms and think not of Doctor and daughter, but husband and wife. Add this to the knowledge that Moffett is the actual daughter of previous Doctor Peter Davison (her opening line a self-referential “hello Dad”) and you have another of those stories which is impossible to watch without our fannish knowledge laying a couple of extra layers of meaning over the top.

*****

So Greenhorne’s Theory (as it now must ever be known) was actually proven right. The Doctor’s character is kind of story proof. The addition of unforeseen family members doesn’t change that. Perhaps we should have known; he’s had a granddaughter before and when she left, it wasn’t as if his character radically changed.

What, then, could be a game changer for the Doctor? I think the closest the series has come to this in the past is his exile to Earth during the Pertwee era. With his planetary travelling curtailed, a hint of frustration and deviousness crept back into his character, which could have developed into something quite dark. The Twin Dilemma too, was another point where something fundamental could have changed about the Doctor, given a more favourable set of circumstances.

Another potential pivot point for the Doctor was The Waters of Mars. Here, the Doctor develops a hubris based on his own powers and dallies with changing the timelines around to suit himself. It all goes wrong and the Doctor sees the error of his ways. But watching at the time, I wasn’t entirely that that was what had happened. I imagined an alternative version of The End of Time, with a Doctor whose newfound appetite for history changing had corrupted him, and made him the despotic ruler of a new twisted timeline, with the Master resurrected as the only being with a chance of stopping him.

What else might have a chance of disrupting the Doctor’s world? Could for instance, he incur a disability of some kind? What would, for instance, a blind Doctor be like for a couple of stories, or even a series? What about a Doctor in a wheelchair? Or what perhaps if he was robbed of his ability to regenerate? Or perhaps robbed of his knowledge of how to pilot the TARDIS, making every trip to a random destination, as per back in the old days.

Surely the big throw of the dice we’re waiting for – and now that Peter Capaldi is leaving it’s a live option again – is what if the Doctor became a woman? It seems we’ve been waiting for this for so long that it’s now become a bigger point of contention that it needs to be. A female Doctor seems increasingly inevitable – whether it will be 13 or not remains to be seen. But even if P-Cap regenerates into another man, that doesn’t necessarily mean a female Doctor couldn’t be experimented with. Surely in a series with Doctor-lite episodes and Christmas specials, there’s ample opportunity to try this out as a one-off and whet the audience’s appetite. Or perhaps a multi Doctor story where our guest Doctor’s played by a woman?

You want a big change to the Doctor’s character? Want to see how mutable our hero is? That’s the big one. It’s waiting in the wings, ready to be deployed at any moment. My bet is that would have a far more palpable impact than giving him a daughter, who’s not really his daughter and who we’ve forgotten about anyway.

Go on, Mr Chibnall. Give Greenhorne’s Theory a real test. I’m dying to see what happens.

LINK TO The Mind Robber: both have credited, but unrelated, Troughtons.

NEXT TIME: Your infantile behaviour is beyond a joke! We play a few games with The Celestial Toymaker.