Another highly self-contained threat to Earth, but I have to admit I'm impressed at the way that they've spread it out across all their regular cast and sets. (Gryffen's house, the Department offices, and those handy sewers again)
Plot: In the sewers, the Department (still with no name but a logo) are trying out an alien vacuum cleaner. This uses a gene-spliced swarm of microscopic organisms to eat anything alive (think of the Vashta Nerada), specifically bacteria, before getting sucked back into the machine again. Well, Darius borrows it to clean up Gryffen's flat with, and in so doing nearly wipes out all life on Earth. He knows the risk, but it doesn't bother him. On some level you have to admire that.
At the story's conclusion, K9 has to suck all the bacteria into himself and then keep it there. Indefinitely. He makes some odd remark about venting it the next time he's on the carbon-life free planet of Atrios, however if we're honest that doesn't look like it's going to happen any time soon. Hang on - Atrios? I dunno, I thought that looked fairly full of carbon-based life when we last saw it back in The Armageddon Factor, but it's been a while since I watched that one.
(just repair the vacuum already)
The main development of note here is that we at last get some progression to the Drake / June storyline at the Department. After the standard bickering and empty threats about whose jurisdiction is whose, at the end of this episode Drake is suddenly reassigned away, and replaced by Thorne, who we last saw as the prison governor back in episode two.
I would like to say that I will miss Drake, and hope that he keeps returning to become one of those popular ongoing baddies with the fans, but I can't really see that coming. Actor Connor van Vuuren has done a fine job with his facials for the character, but his dubbing has been a disaster in just about every scene. Let's be clear here - this show uses dubbing like oxygen, and it makes pretty well all of its actors sound hideous, but someone has to come out sounding worst, and randomly that position has fallen to van Vuuren.
I sincerely hope that in the actor's next job the director gives him some proper microphones on set. Like the character, that's a man's career they're toying with there.