The story, by William Gallagher in his first four parter for the range, is something of an odd one. There's some good atmosphere and very nice ideas, but the dependence on technobabble and some fairly silly characters drag the thing off course before the end.
The story opens with the Doctor and Flip visiting a post Sontaran Experiment Earth where they accidentally get transmatted (there's a drinking game to be had in the amount of time variations on the word “transmat” are used throughout, but no one bar Oliver Reed would survive it) to the frozen wasteland of Loch Lomond in what was Scotland. The Buchman family have returned to reclaim the “colony” (for which read cottage basically) they'd been forced to abandon 15 years earlier due to orders from Nerva City, but dark secrets from their past, along with a huge amount of frozen Wirrn out in the ice, threaten to destroy them all.
With the Wirrn discovered to have eaten the work crew that were supposed to set up the house for the Buchman's and the Transmat booth having been switched off by Nerva City to encourage the colonists to stay out there it's down to Flip to take the only transport, a microlight belonging to the families dead son, to try and reach the local transmat substation out on an island in the middle of the loch. She crashes, and is attacked by Wirrn but makes it to the substation where she manages to transmat to Nerva City.
Meanwhile, it turns out that head of the family Roger, a transmat expert, had known about the Wirrn all along as before they left previously he'd accidentally transmatted his son Iron into the body of one. His attempt to use the DNA from his daughter, Toasty (it's not entirely clear, but I really hope this is a nickname) to separate Iron from the Wrrin Queen. All he succeeds in doing however is transmating her into nowhere, basically running through the machine constantly (and possibly hanging out with Scotty in there). Mother Veronica is already ready to crack after her seemingly dead son starts chatting to her on the radio, and is just about on the verge of a complete breakdown at this news.
The Doctor realises Iron is really dead and the Wirrn is just using his memories, and that the creature is keen to use the transmat. He tries to bring Toasty back using Roger's DNA for a lock, but the transmat rejects this as he turns out not to be her real father.
Over in Nerva City Flip tries to warn the City's leader about the Wirrn, but the only people who believe her are Sheer Jawn (mainly because he's Toasty's dad) and his partner Dare. Jawn is keen to rescue his daughter, and transmats over to the loch, but unfortunately picks exactly the wrong moment as “Iron” has wound up on the pad as well. Now with two human minds inside it the Wrrin starts really mind fucking with Veronica by switching between persona, before realising that as Sheer was a transmat expert as well and thus it no longer needs Roger, and so it kills him before setting up the machine to Transmat copies of it everywhere there's a receiver.
This works, and soon Nerva City is overrun with Wirrn. The Doctor is at least able to transmat Toasty back using Veronica's DNA, and though the mother is understandably keen to kill all the Wirrn he uses Dawn's help to set up a system that will transmat all the Iron/Sheer/Wirrn duplicates back into the lake and freeze it again. Leaving humanity some time to come up with a way of making peace and dealing with all the eggs that have been laid in unwilling human hosts. He then basically does a runner before the more likely outcome of the humans brutally killing all the Wirrn can happen.
Well, I said there was a drinking game over the word transmat didn't I?
The early atmosphere is incredibly tense, we know what the monster is and where they are (from the cover), so the slow creepy build up in the Scottish desolation works wonderfully. It's also nice to have a genuinely dysfunctional family in Doctor Who that don't have all their problems magically sorted out by the end. Neither parent is very likeable, but they are interesting and Roger being killed by something with the memories of his son is a very nasty moment.
But all the guff about transmats ultimately makes very little sense (there's no explanation as to what caused Iron to wind up in a Wirrn, I'm assuming it was an accident but from all the play tells us it might have been deliberate) and basically means the play drowns in people solving problems by pressing buttons. It effectively kills all the good work of the set up because every time something interesting happens, such as thousands of Wirrn swarming all over Earth, we cut to the Colin Baker trying his best to convey staring at a computer in the audio medium whilst talking complete nonsense.
It's also very odd that we have a monster that already absorbs the memories and the voice of those it gestates in, but the play contrives to create an artificial situation for this to happen. About 5000 uses of the word transmat could have been avoided if Iron had just been turned into a Wirrn Queen in the same way as seen in The Ark in Space.
The play also fails at conveying time properly, meaning Flip's bravery in flying the microlight seems pointless as the Doctor and Veronica seem to walk to where she crashes just about as quickly as it took her to fly there.
The big problem is Toasty however. A very strong contender for the single worst written and performed guest character in the whole of Big Finish. There a couple of lines stating she's 19 (her 20th birthday is soon and Iron “died” 15 years ago when she was 5) but she comes across as a mentally backwards six year old, with a very childlike way of talking and a deeply irritating high pitched voice.
It's been suggested over on Gallifrey Base that the character was originally written as being much younger but this was changed to avoid using child actors. This is plausible, but if it were the case the script really needed to be changed more than just altering the dating slightly. And it doesn't explain why actress Tessa Nicholson decided to play her in such a bizarre way (and the making of extras firmly establish she knew she was playing an adult). Between the odd vocalisation and a few lines that feel like they're left over from an earlier draft where she had some more sinister purpose it initially feels like there's going to be some immense twist where it turns out she's really half Wirrn or something. But no, she's supposed to be a perfectly normal adult. Just one you want to see get eaten by a monster.
The Doctor's morality is somewhat off as well. Now, Six tends to be the most pragmatic incarnation and there's nothing wrong in theory with him considering the Wirrn just as worthy as the humans but there's an immense double standard at work. If the Wirrn have the right to do what they do naturally (which lest we forget means planting their eggs in humans so they get eaten from the inside out) in order to survive, surely the humans have as much right to fight back against them to protect their own way of life? You can't be pragmatic about the natural order for one species but not the other (and the possibly mitigating factor that humans waging war upon them and driving them to the brink of extinction was the reason the Wirrn came to Earth in the first place doesn't even warrant a mention). It winds up feeling like may Sillurian stories do, where the monster could go around eating babies and raping kittens with the Doctor's approval whilst any human who goes “Well, gee, maybe we should like fight back” is instantly slapped down for being a damn dirty ape.
Despite The Fourth Wall just about managing to stay on the right side of very good the Flip trilogy has overall been a disappointment. Both the first and the last play had good ideas that could have worked brilliantly but failed somewhat in the execution and, after a weak 2011 trilogy for him as well that means it's been a couple of years since we had a good run of main range stories for Colin.
And as for Flip herself, she's now at the stage where I can live with future appearances but equally I'm not chomping at the bit for them. I'm somewhat glad the 2013 trilogy has already been confirmed as featuring Mel. And that says it all about how one dimensional and trite Flip has been overall, I'm now in a position where Mel of all people seems a more desirable character (Big Finish have done good work with her, but tat the start of the year the idea of three plays on the trot with her would have curled my hair. Now I welcome it).
There's still enough good tension in the setting to stop Wirrn Isle being a complete write off, but if it were transmatted away from my collection I wouldn't be too upset.
[Two and a half humans accidentally transmatted into a Wirrn out of five]