You can tell a Steven Moffat script. The writing is full of clever ripostes and zingy one-liners, delivered at just the right moment, with just the right amount of sardonic wit. But my favourite line of dialogue from (the thankfully less grisly than it sounds) The Girl in the Fireplace is much simpler and more functional. It’s this:

ROSE: Why her?

That line comes in the middle of a standard mid-episode exposition scene, albeit in an episode with a more complex premise than most. The Doctor (David Tennant), Rose (Billie Piper) and Mickey (Noel Clarke) have discovered a spaceship in the far future, linked to a number of times and places in 18th century France. Present in each of these locations is Reinette, AKA Madame de Pompadour (Sophia Myles), at different stages in her life, and a cohort of clockwork androids are crossing from future to past to monitor her.

In this scene, our TARDIS crew and Reinette have cornered one of the robots and are interrogating it on what’s going on. They’ve found out that the robots have cannibalised the ship’s crew for parts (still 21st century Who’s most gruesome plot development) to repair the immobilised spaceship, and they want Reinette for a mysterious, but crucial, final component.

The pivotal question is why, out of all the people in history, do the robots want Reinette? The question must be asked, but think about Moffat’s choices about which character should give voice to it. He could give it to the Doctor, but he’s already carrying the bulk of this scene. He could choose Reinette and change the question to “why me?” It would be a perfectly understandable question for her character to ask.

Instead, he gives it to Rose. And in a moment which shows what a smart and subtle actor she is, Billie Piper manages to drench those two words with subtext. Yes, she wants to know what the robots find so fascinating about Reinette, but she’s really asking why this fascination has spread to the Doctor, whose romantic interest the girl in the fireplace has piqued. Why her, she’s asking, and not me?

*****

Moffat has said that this episode is “the one where Doctor Who gets a girlfriend”. Which cheerfully ignores the fact that he already has a girlfriend in Rose. But then the Doctor and Rose have never been what we might call “official”.

Truth is, Rose never knows where she stands with the Doctor. Yes, she’s snogged him but both times were under extraordinary sci-fi infused circumstances: once he drew a bundle of time energy out of her to save her life (via her lips) and the other time she was possessed by a notorious vamp (and not responsible for the actions of her lips). No commitment has been made by either party. Which is where the romantic insecurity sets in. Mickey teases her about it this episode when listing possible suitors for the Doctor in Sarah Jane Smith, Cleopatra and now, Madame De Pompadour.

None of which would matter if Rose felt secure in her relationship with the Doctor, but she’s always played it pretty casually too. She’s kept Mickey on the hook for long enough, keeping her options open. So she doesn’t really have cause to complain when the Doctor takes up with Reinette in record speed. But it obviously bugs her. It’s pretty clear throughout this episode that Rose is thinking, “what if he invites her to come with us? Or what if he decides to stay with her?”

Why the Doctor is suddenly so taken with Reinette is more difficult to work out. Sure, she’s a beautiful woman (probably) but the Doctor’s hung out with plenty of them before and never hooked up so quickly (that wild New Year’s Eve back in 1996 excepted). It’s tempting to think that he’s just so unused to romantic relationships that he’ll latch on to any girl who’s deigned to kiss him.

To be fair, Reinette is remarkably smart, capable and unafraid to take what she wants. He’s not just fascinated by her, but by the mystery of why this troop of ticking androids wants to plug her into their spaceship. (Here we see one of Moffat’s favourite plot lines beginning; a woman as an intriguing puzzle for the Doctor to solve.) Sure, Rose is the girl next door, but Reinette’s an enigma wrapped in a ball gown.

If the Doctor’s aware that he’s causing Rose consternation, he certainly doesn’t show it. Can he really be so blind to her feelings? Doesn’t he know that they’re quasi boyfriend and girlfriend? He’s clearly no stranger to romantic jealousy. Look at how snarkily he tells Louis (Ben Turner) that a lord of time trumps a king of France.

I think it’s more likely that with Rose’s determination to keep her options open with Mickey, the Doctor’s assumed that this whatever-it-is with her is not necessarily going to be monogamous. Sadly, we never got a serious suitor for Rose to find out whether he’d react in the same way (Captain Jack kinda made him jealous for a bit but I’m not really sure that counts. Given that he turns out to be turned on by just about everyone he meets).

It’s sometimes pointed out that the Tennant Doctor and Rose don’t treat other people very well, so ensconced are they in their bubble of love. But The Girl in the Fireplace shows that occasionally, they also don’t treat each other very well. It’s pretty hard to ignore the fact that the Doctor heads off to “dance” with Reinette mid-episode, with no thought as to how this might make Rose feel. Cheating on his girlfriend with his new girlfriend. The Doctor as a cad – that’s something genuinely new.

Rose and Reinette end up representing the two types of woman our 21st century Doctor, with his new interest in romance, will end up flirting with over the next few years. On one hand, you have the sassy girl next door types: Rose, Amy, Clara and Martha. On the other, you have mythic, powerful, uber women: Reinette, River Song, Tasha Lem, Queens Elizabeth and Nefertiti. Both are idealised feminine archetypes, though at opposite ends of a spectrum of hetero male fantasies. It would be interesting to see the Doctor fall for someone who sits more realistically between these two ideals.

*****

I have a favourite shot in The Girl in the Fireplace, to go along with my favourite line. It’s the final one where we at last find out why the clockwork robots are as fascinated in Reinette as the Doctor is. As it turns out, the answer to Rose’s question was there all the time, obscured by a particularly unfortunate piece of TARDIS parking.

Why her? Well, the ship is named after Madame De Pompadour. In using the very last moments of his story to put the last of its jigsaw puzzle pieces in place, Moffat underlines that the big events of our lives – who you fall in love with, who falls in love with you – depend to a large extent on coincidence and will always remain, at least in part, a mystery.

LINK TO The Woman Who Lived: Both titles refer to mysterious female guest stars.

NEXT TIME: Family history and time travel? Very tricky. Break out the shop bought cake for Demons of the Punjab.