In the middle of The Unicorn and the Wasp, there’s a standout scene where the Doctor (dishy David Tennant) is poisoned after drinking a spiked lemonade. Cue an outrageous slapstick scene where he staggers to the kitchen to ingest a miscellany of random ingredients to order to “stimulate the inhibited enzymes into reversal” (hmmm, sounds sciencey). First ginger beer, then walnuts, anchovies… all accompanied by a frantic game of charades with companion Donna (flapper, not slapper, Catherine Tate). The punchline to this elaborate joke of a scene is the delivery of a smooch from Donna to shock the Doctor’s system into expelling the noxious substance from his body in the form of a foul gas. From his mouth.

In the middle of this sly and witty murder mystery, here is a moment of pure slapstick. The elaborate physicality, the overplayed reactions, the knocking of over of all sorts… in fact, this could be Doctor Who’s ultimate slapstick moment. (Sorry, that sounds like one of those cheap clip shows which periodically materialize to eat away your time. “Doctor Who’s top 5 ultimate slapstick moments!” an excited voice over would announce, over a tinny version of the theme music and miscellany of publicity photos of past Doctors, flying at the screen).

It doesn’t get mentioned much, but slapstick has a proud history in Doctor Who, despite 80s producer John Nathan-Turner’s much stated opinion that comedy in the series was about wit, never slapstick. He was true to word, at least for the first few years of his producership. Later on, he presided over Bannermen being pelted by jars of honey, milkshakes being poured over café goers heads and the Kandyman being immobilised with soft drink… so he must have got over that particular bias.

JN-T used to talk disdainfully about slapstick in order to differentiate his era from the show as produced by Graham Williams, which fan lore held that was altogether too silly. But slapstick had long been part of Doctor Who’s approach: The Romans had comedy fisticuffs, The Seeds of Death a dash through a hall of mirrors and the Doctor smothered in a deluge of foam. Even the po-faced Pertwee years found a few minutes to run over a tramp with a hovercraft.

It was Russell T Davies, though, who truly revelled in slapstick moments in Doctor Who, from the Doctor and Rose’s madcap dash from the Hoix in Love & Monsters, to the Doctor’s expulsion of radiation into his shoe in Smith and Jones. Davies was never afraid of making the show look silly, in the way which seemed to terrify Nathan-Turner (at least until he dropped green gunge over Balazar’s face in The Mysterious Planet). He knew that slapstick was a delightfully sweet treat within an otherwise dramatic episode.

The Unicorn and the Wasp is something different, though. It is, as Davies acknowledged in The Writer’s Tale, his first attempt at an all-out Doctor Who comedy and slapstick is only one of the tactics used, in a kind of mixed lolly bag of comic approaches. (Though for a comedy, it has some grim undertones. It does, after all, feature an alcoholic mother who loses both her sons on the same day. Fun times!).

For a start, there’s pastiche. This is not just a Doctor Who version of an Agatha Christie story. It a Doctor Who version of the television adaptations of Agatha Christie novels. You’ll know them as handsome Sunday night viewing: large casts, beautiful costumes, stunning locations and faithful recreations of times gone by. The story’s structure lifts familiar scenes from these adaptations – the dinner disrupted by murder, the gathering of suspects together for the big reveal. Even the filmic trappings of murder mysteries – flashbacks, spinning newspapers et al – are employed. At one stage, Donna’s eating popcorn like she’s watching the whole thing on TV. We know how she feels.

Then there are in-jokes. The constant quoting of Christie book titles. Donna’s pre-knowledge of Christie’s work. The deliberate evocations of, of all things, Cluedo. And the moment where Donna questions why Christie is experiencing events similar to her own plots. It’s a pleasant surprise when it turns out the butler didn’t do it. It’s so self-knowing it hurts, perhaps the most self-knowing the show has been since the infamous moment in Dragonfire (itself no stranger to slapstick), when a character quoted a Doctor Who academic book about the “semiotic thickness of a performed text.” Which in turn only added to Doctor Who’s semiotic thickness.

On top of all this, it’s just funny. Barely a scene goes by without a joke, verbal or visual. You can choose your own favourite, but mine’s how Davenport (Daniel Hill) sheepishly pokes his head out of Roger’s (Adam Rayner) bedroom door during the corridor scene. But that’s closely followed in my affections by:

DONNA: It’s a giant wasp.

DOCTOR: What do you mean, a giant wasp?

DONNA: I mean, a WASP that’s GIANT!

What I’m getting around to saying is that Doctor Who has often used comedic techniques in the past, just never before all at once. And thinking about this episode and how it mixes genres and comic forms made me ask: when does it stop being homage and start being spoof?

Doctor Who skirts this line occasionally. Other examples include Delta and the Bannermen, City of Death, The Gunfighters and The Feast of Steven. But I think spoofs (spooves?) prioritise the gags over telling a consistent, logical story. We’re yet to have the Doctor Who equivalent of Flying High for example (no, Time-Flight doesn’t count) because Doctor Who is never just a string of jokes. And The Unicorn and the Wasp is certainly more than a string of jokes; the weaving in of Christie’s story of lost love and self doubt gives the story a contrasting element of pathos.

But it’s an unusual experience watching this constantly self-referential story, so eager to invite us all to be in on its extended joke. Christie’s personal crisis aside, there’s barely a moment which isn’t winking conspiratorially at the viewer. It’s Doctor Who mimicking a TV version of an Agatha Christie novel, while saying to its audience, “Look! This is Doctor Who mimicking a TV version of an Agatha Christie novel! With a big slapstick routine and a WASP that’s GIANT!”

The semiotics of a performed text has never been so thick.

LINK TO The Angels Take Manhattan: Talking of genre… as this story takes up murder mystery, The Angels Take Manhattan is Doctor Who doing film noir.

NEXT TIME: Talking of self-knowing references… Even the sonic screwdriver won’t get us out of this one. It’s time for The Invasion of Time.