This week the Doctor and Donna materialise in one of the narrowest and most tediously predictable literary cul-de-sacs ever to take up space on a bookshelf – the 1920s English murder-mystery. Yes, this ‘historical’ episode is not only rendered in the style of Agatha Christie, but features the author herself as a principle character.

Just as the series 4 ‘holiday’ episode must surely be coming up (last year it was Stephen Moffat’s award-winning Blink), so The Unicorn and the Wasp proves to be this year’s budget-slashing, wardrobe-box raiding period filler (rather than thriller) which has clearly surrendered the majority of its budget to the effects bills for Fires Of Pompeii.

The Tardis materialises with its reassuringly unaltered ‘shushing’ sound onto the lawn of high-society toff Felicity Kendall, just as she is expecting guests, including her beloved favourite author, Agatha Christie (played with suitable English demure by Fenella Woolgar).

Donna’s attempts at a clipped English accent are amusingly stifled by the Doctor, but it’s clearly a great thrill for her to meet the arch-hack and literary bore herself, as it presumably is for writer Gareth Roberts to get to explain Christie’s amnesiac 10-day disappearance in 1926. So it’s all cucumber sandwiches and tea. For about one minute…

But this is new Who, which will not surrender its dizzying pace even to the languid tone of a classic murder-mystery. The niceties promptly transform into a busman’s holiday for the intrepid duo, as one of the cluedo-esque guests is killed in the drawing room to the amusing exclamation ‘Why didn’t they ask…heavens!’ (yes, very good).

Before you know it, the Doctor has flashed his psychic paper (a plot-solver that is becoming arguably lazier than the sonic screwdriver) once more and he and Donna are flourishing every clichéd accoutrement of the genre and having a bit of fun with it too.

With Donna relishing her role as Tennant’s ‘Watson’, it’s left to the time-lord and the authoress to investigate the alarming avalanche of murders that follow the first. Since Doc has found some clearly alien 'morphic residue' at murder number one, there’s no question of calling the real police in - which is handy for maintenance of the cliché.

Trouble is that Christie is not up to the job, suffering a crisis of confidence in the wake of discovering her husband’s infidelity, but Donna, the new ‘heart’ of Doctor Who (and at least doing a less irritating job than Deanna Troi) is on hand to take her aside and commiserate.

In the meantime, it’s looking awful likely that the ghastly murders are being committed by a medium-quality CGI wasp the size of a motorbike, and one that Lady Felicity Kendall may know a fair bit more about than she is letting on…

The highlight of Unicorn is the Doctor’s Casino Royale-style efforts to detoxify himself when he realises that he has been poisoned with arsenic by the murderer among them, which necessitates the administration of a nasty shock. At this point, Catherine Tate obliges with a huge frenchy. I must admit, that certainly qualifies in my book.

The running joke about the Doctor and Donna ‘not being a couple’ is beginning to look a bit leggy, and also has the worrying ring of anti-truth. Oh Gawd, no…

At the earlier introductions on the lawn, Tennant introduces himself, customarily, as ‘the Doctor’, and once again no-one asks the inevitable question, ‘Doctor Who?’. If he needs psychic paper to convince them that he is a policeman, why does he need nothing at all to convince them (and legions of other earthlings) that it’s perfectly normal to have a title and no name? I can’t ever recall getting away with introducing myself as ‘the Mister’…

Predictably Donna ends up telling Christie about future events in her life, and novels that she has not written yet, attempting to worm her way into the copyright, but who would the publishers have paid half a century before Donna was born?

The cross-species breeding between Kendall and the wasp-creature, whilst only alluded to, is a fairly mind-boggling leap of imagination, whereas the 'firestone' locket proves to be yet another super-powered Who McGuffin, this time capable of inculcating a life-time's worth of experience quicker than you can...well, clone a time-lord, and the creature letting Christie 'go' at the end seemed like a tacked-on 'awww' moment. Are these now requisite for every episode?

The retro-fitting of the austere Agatha Christie literary-landscape with such modern touches as a 'gay assignation' proved an amusing update in the flashback sequence, but the final human>wasp transformation was a disappointing cross-fade that remains as hokey a technique now as when it was (frequently) used on Who in the 70s and 80s.

Director Graeme Harper is a veteran Who-hand, having started out as production assistant in the Pertwee era and now successfully adapted to the increased tempo of the RTD age in a great number of stories for the new series, most recently in the excellent Planet Of The Ood, and he continues to be adroit at getting decent performances out of regulars and guest cast alike.

Writer Gareth Roberts is on famiiar territority in Unicorn, as he got the Doctor together with The Bard in last season's The Shakespeare Code, as well as being a Sarah Jane Adventures protégé, and contributing ‘Tardisodes’ and the Attack of the Graske videogame for the Beeb's Doctor Who website in 2005, and writing six Who novels for Virgin publishing. I’m guessing that he is more fond of Black Orchid than I am, and relished the chance to put the Gallifrean into this environment again.

Roberts’ episode does relatively little harm or good to new Who, as it stands so utterly outside of the flow of series 4, and if you like the subject-matter, then you might like the episode. It just seems a shame to waste a period-story on the trite balderdash of Agatha Christie, and it’s an absolute obscenity to suggest that she will be the best-selling author five billion years hence. And that’s supposed to be a happy ending?

We’re all expecting more of the highly anticipated Stephen Moffat two-parter which is coming up after eurovision, Silence In The Library and Forest Of The Dead (could RTD not consider adopting the ‘parts I and II’ etc convention? It worked very effectively for nearly thirty years and caused less confusion). Look out for the shadows….

Check out Martin's review of The Doctor's Daughter here. Simon's review of it is here.

Doctor Who interviews at Geek...

Peter Davison
Louise Jameson
Elizabeth Sladen
Sophie Aldred
Nick Briggs
Nicholas Courtney
James Moran