I think a key challenge facing Steven Moffat and the other talented folk who make Doctor Who is this: how to find new ideas for a fifty plus year-old program. In fifty plus years, you can tell an awful lot of stories. Pity the new scriptwriter who pitches a storyline to the Moff only to hear ‘they did that in 1975’ or ‘we did that last year’ or ‘they did that in 1975 and we did it again last year.’

But genuinely new things to do with Who are rare and glorious. Take The Doctor’s Wife, for example. It’s sandwiched between The Curse of the Black Spot, which features pirates (done) in a space hospital (done) and some malfunctioning technology (oh dear lord, so done) and The Rebel Flesh with doppelgängers (done), grown in vats (done) in a grimy industrial setting (done) with a duplicate Doctor (the donest of done). But The Doctor’s Wife has a uniquely novel idea – the TARDIS becomes a woman – at its core.

When a new idea comes along, it’s refreshing and energizing. In Series 9, we’ve had a few of these trail blazing episodes: Sleep No More with its challenge to narrative structure, Face the Raven with its Potteresque secret world of aliens hidden in a London alleyway and the virtuosic one-hander Heaven Sent.

But Series 8, Peter Capaldi’s first season, is a game of two halves in this respect. The first half of the season feels safe and familiar. Deep Breath has the Paternoster gang, dinosaurs in London and the bad guys from The Girl in the Fireplace. Into the Dalek is The Invisible Enemy crossed with Dalek. Robot of Sherwood‘s not only that year’s celebrity historical, it’s a rerun of The Time Warrior. The Caretaker is a second attempt to recapture the success of The Lodger. Even Listen is a classic Moffatesque kids-afraid-of-the-dark, timey wimey conceit. So far so familiar.

But half way through, the whole season wakes up. Killer spiders on the Moon, which isn’t the Moon. A Mummy on the Orient Express. Forests reclaiming the earth. Love ’em or hate ’em, they are things the series hasn’t done before. Some of them are mad, outrageous things, but they are genuinely new. And smack in the middle of it all, Flatline, a story about two-dimensional monsters invading our three-dimensional world. Another startlingly new idea.

*****

Flatline feels fresh thanks to its 2D villains, never done before in Who. Conceptually, they are difficult to latch on to. Essentially, they are creations of clever camerawork and CGI. Kids will never be able to draw a Boneless as they might draw a Zygon or a Dalek. They’re monsters which truly liberate Doctor Who from its seemingly endless need for men in rubber suits.

And their non-physical characteristics are just as oblique. We never hear them speak. They don’t even get a name until the episode’s climax. We get no insight into what they want, how they infiltrated our world and no clue as to why or how they can leech away at the TARDIS so effectively. They’re just there, ready to climb out of any flat surface, and stumble drunkenly towards you, your eye never quite getting a true sense of their shape. Writer Jamie Mathieson knows that fear of the unknown is the most potent of all, so he tells us next to nothing about these silent killers. In the end, the Doctor has to make up a name for them, just so we can have something to list in our big book of monsters.

Another way this episode feels new is in its supporting cast, drawn from a gang of community service workers. Local tearaway Rigsy (Joivan Wade) is a graffiti artist, which adds a nice thematic layer in a story about 2D monsters. He and his fellow workers might be more a photogenic lot than your average do-no-gooders but still –  it’s rare for Doctor Who to draw its supporting cast from such unpolished stock. Misanthropic old Fenton (Christopher Fairbank) is familiar enough, but Mathieson makes an effort to make his characters feel different to type. Even bit player Bill (James Quinn) the train driver isn’t what you’d expect. As soon as Clara (Jenna Coleman) asks him to ram a blockage with his train he says, “Is this official? Because I’ve always wanted to ram something.” Which now has me worried about train drivers generally.

But like series 8 as a whole, it’s not all new stuff in Flatline. The Doctor trapped in a shrinking TARDIS happened back in Logopolis. When unpleasant old bastard Fenton survives the episode intact, it’s a repeat of what happened to Rickson in Voyage of the Damned. It’s difficult for a story to feature monsters in underground train tunnels and not evoke The Web of Fear. Most familiar of all though, is the trope of the companion assuming the role of the Doctor.

*****

In Flatline, Clara plays at being the Doctor while he’s trapped in his box. It starts off as a bit of fun, with her taking the opportunity to take a few humorous shots at the old man while he can’t do anything about it (my favourite is when she’s asked what she’s a Doctor of, and she replies “I’m usually quite vague about that.”). But soon enough she’s taking charge of the situation, and making decisions to help keep people alive. The Doctor’s none too pleased with the ruthless pragmatism she exhibits. It’s a trait he’s comfortable with in himself, but finds less palatable in Clara. And as we now know from Face the Raven, it’s a trait which gets her killed.

“Someone’s got to be the Doctor,” said Rose back in The Christmas Invasion, the story which kicked off this whole ‘companion as Doctor substitute’ thing. Since then Martha has had to save the entire world while Doctor became an wizened little elf and Rory’s had to act up in the role while he’s been quarantined in the TARDIS (“You’re turning me into you!” Rory bawled on that occasion). And Donna became a human Doctor hybrid. It seems new Who’s fascinated with seeing how a companion would fill the Doctor’s shoes, which the old series rarely did (though as we’ve seen recently, it let the companions dress up as him).

Which as far as I can see leads us to one logical step Doctor Who has yet to take, which is for an episode where the Doctor actually inhabits his companion’s body, and vice versa. It would give us a chance to sample what a female Doctor would look like. If it’s not a female companion, then maybe it’s Missy who might might play the host body. And if we’re able to overlook Freaky Friday, I think that qualifies as a genuinely new idea.

UPDATE: 9 Dec 2015. I suspected I might feel like updating this article post Hell Bent. And sure enough, in it, we saw Clara’s transformation into a surrogate Doctor become complete. But I think few of us would have predicted she’d end up in an incongruous costume, piloting a faulty TARDIS on the run from the Time Lords, with a pretty female companion. As the Doctor aptly put it, “Clara who?”.

Interestingly enough though, this is not so much a new idea, as it is the long awaited realisation of a previously mooted one; that upon leaving the show, a companion could become a proto-Doctor. Years ago, it was suggested that in the never made Season 27, Ace may have left the Doctor to join the Time Lord academy. As it turns out, we’re fortunate it never actually happened. Otherise the Moff might have dismissed that notion with a desultory “they did that in 1990.”

LINK to Dragonfire: Both feature guest stars called Quinn; Patricia Quinn in Dragonfire and James Quinn in Flatline. Tenuous link alert!

NEXT TIME… All the way from Metebbalis 3, it’s Hide.