If you cut up any number of Russell T. Davies’ ‘invasion’ scripts for new Who and make confetti of it, don’t let Helen Raynor sweep up afterwards, as the fragments are likely to end up as a medium-budget two-parter featuring – and devaluing – the classic villains/monsters of the series.

Raynor performed this disservice towards the Daleks as writer in the series 3 entry The Daleks In Manhattan/Evolution of the Daleks, and has made an even less convincing reprisal for the Sontarans in the two parter concluding with tonight’s The Poison Sky (following on from last week’s Sontaran Stratagem).

I can only conclude that former series script editor Raynor got this particular gig because of favourable numbers for the awful Manhattan debacle, but a Dalek plotline is a gilt-edged gift to a Who script-writer, and should go to the best of the best as a reward for innovation and invention elsewhere in the series.

Anyway, some matters pertaining to the two-part Sontaran saga were presumably beyond Raynor’s control or ambit, such as the fact that this particular iteration of Sontarans are amongst the most short-arsed bipedal villains ever to threaten Earth. The casting of diminutive Christopher Ryan as General Staal seems to have triggered an equity alert for the vertically-challenged, and this did nothing to increase the general sense of alien menace. In addition, the old mistake of voice-miscasting for the ancillary Sontarans lent an unintentionally comic note to the proceedings, as the dulcet tones of Surrey accountants issued from the well-done potato-head make-up.

Poison Sky begins with the launch proper of the Sontarans' evil strategy to pollute the Earth’s atmosphere with deadly gas that will prove fertile material for a race of new Sontaran clones (this being their efficacious - if impersonal - method of reproduction).

After a fair bit of tedious Donna-and-her-mum sparring (and the usual excellent contribution of Bernard Cribbins as Donna’s grandfather), the Doctor sends Donna to baby-sit the Tardis in safety, only to find that she has been teleported with it to the Sontaran war-base in Earth’s atmosphere. Stranded, and with an imminent countdown to nuclear disaster ticking away, our Gallifrean hero strides past those stiff-shirted UNIT types and contacts Sontaran HQ for a bit of jaunty badinage, baiting and miscellaneous attempts to get a message to the stranded Donna.

At this point, we get a flash of Billy Piper trying to contact the Doctor, but without any sound. Let’s be grateful for the latter fact, and for how brief her appearance was, and let’s enjoy whatever respite (may it be protracted) that RTD gives us from the insufferable Rose before her scheduled return later in the series.

Back at the UNIT headquarters, all current hope of seeing the excellent Nicholas Courtney reprise his role as Alistair Lethbridge-Stuart vanishes in two puffs of dialogue that reveal the ageing brigadier to be stranded in Peru. Meanwhile the Martha Jones clone is casting arch-looks at the nuclear-strike switches, but not really doing anything meaningful beyond reporting the Doctor’s plans to her Sontaran masters.

Over at Luke Rattigan’s Koresh-style brainwashing factory, the future Adams and Eves are showing clear signs of dissent, unimpressed with the boy genius’s plans to start the human race up again on some (as it turns out, fictitious) planet called Castor 36, an opportunity promised to the nasty little collaborator by Sontarans seeking his help in murdering 6 billion people.

The potentially interesting SFX potential of the super-smog itself was dispersed in a series of matte-painting style tableaux – nicely done in themselves, but lacking the support of street-level smog mayhem.

When the Sontarans actually get off their arses and start performing the basic duty of Doctor Who villains for over forty years – striding menacingly down corridors – UNIT threatens to be overwhelmed. The Doctor, faced with the pig-headedness of the UNIT brass, embarrassingly resorts to the classic Aliens line ‘Get them out of there!’, which Hollywood ran into the ground in the ten years following the release of James Cameron’s action classic.

Anyway, we’ll let the Doctor get away with it (he may not be a cinema-goer, after all)…except that he then says it again but VERY LOUDLY. Tennant’s time-lord is getting very shouty these days, and he’s not quite as chilling as some of his predecessors when he does it.

Donna manages to contact the Doctor, who guides her through the basics of defeating Sontarans (hit the back of the neck – is there no Sontaran technological remedy for this?) and through the war-station, where she is tasked to re-establish the teleport links.

Out and about in gas-masks with UNIT, Tennant throws in a pretty funny reference to the excellent Empty Child Ecclestone story, before UNIT's Captain Scarlet-style Cloudbase platform blows a bit of the local gas away.

Some good SFX finally show up when the Doctor uses a kind of cosmic firework to burn the deadly Sontaran clone-gas off the planet’s atmosphere, which clears itself like a (very big) lit fart. By now the savant Rattigan has been abandoned by his doubtful followers, but redeems his genocidal tendencies in an act of self-sacrifice that saves the Doctor and the Earth from a more standard military-style Sontaran invasion. And then it’s all over, thank God.

Martha Jones, having bonded with her repentant clone and watched the doppelganger die (to the usual strains of lush and overbearing music) ends up on board for a few more episodes, as the Tardis is hijacked before she can make a poignant exit.

Next week, we get to meet to meet the Doctor’s daughter (and presumably Susan Foreman’s eventual mother), as played by Georgia Moffett, the daughter of fifth Doctor Peter Davison.

The Poison Sky was a very poor showing for the canon, a gaseous soup of recycled elements from the last three series, with poor characterisation and plotting. Also, I’m not sure how I feel about UNIT having tech quite that ‘sci-fi’, or about these battles for Earth being fought in the public gaze, since the show will later require extra gallons of post-modern irony to cover the fact – and Who is already over-saturated with it.

Perhaps a future episode will come up with a Men In Black-style ‘memory zapper’, so the traumatised denizens of Earth can get back to worrying about terrorists again. If it erases the memory of this Sontaran adventure, my own sunglasses will be staying in my pocket for the event.

Check out Simon Brew's review of last week's The Sontaran Stratagem.