Roots: Brass. The Doctor quotes Oscar Wilde ("I can resist anything but temptation") and paraphrases from The Merchant of Venice Act III, (i) ("If you prick us, do we not bleed?"), Dante's Inferno ("you can be my Virgil") Jerusalem - particularly Blake's "dark satanic mills" versus "green and pleasant lands", but also Hubert Parry's musical adaptation (see: Goofs) Stretton paraphrases Mark 26:52 ("I lived by the machines, but now it seems I will die by the machines.")
Goofs: Stretton sings Hubert Parry's 1916 composition of Jerusalem nearly fifty years before Parry set it to music.
Dialogue Triumphs: "Awkward situations are the Doctor's bread and butter"
"Doctor, there's something going on in the ticket office - everyone stopped to take a look!"
"-Oh, they've probably acquired a new hat stand and it's a slightly unusual colour. Small-town mentality."
"I mean, I'm not being racist, but-" "-Good!"
"First - shut up, secondly - give me the sack. Thirdly, go back to work before I'll be the one giving you the sack."
"Shoot first, saves you having to ask questions later."
"If I cut you, you will not bleed. If I hit you in the nose, you'll probably go 'clong'."
Dialogue Disasters: "It's a hand. A human hand" (does Townsend know of other kinds?)
Double Entendres: "Keep it well oiled, replace its moving parts regularly - should give you years of steam-driven pleasure."
Continuity: Carlia is the home world of the Carlians, a race that is scaly, tailed and red-skinned and therefore demonic in appearance. Carlians have purple blood and a low pain threshold. Their home world appears to be drier than Earth, judging by Belfrage's complaint, and has at least three pleasure moons - frappe-daiquiris are drunk there.
Belfrage, a Carlian disguised as a human using a 'patch', arrived on Earth four years ago in his Astropod, but became stranded after it was stripped (his other means of transport is a Doren capsule but it doesn't appear to be spaceworthy either). He bought the Catalyst (an emergency device used to advance samples of local technology to a sufficiently useful degree for one's rescue) off a Vislak loss adjuster. Vislaks have fingers like sausages and at least two faces.
The Doctor has seen Inhibitors before - they are devices primarily used to deter local populations from technological reliance, even invoking technophobia. Lots of races use them to either interrupt technological progress or otherwise sabotage enemy advancements. They are, of course, illegal.
Earth salt is a delicacy on some alien worlds, but because Earth is pre-first contact in this time period trade is illegal and punishable by detention on a penal asteroid.
The Doctor is tipped off to the presence of a small-scale terrain enhancer by (among other phenomena) the traces of an aniseed smell to the air - a by-product of the enhancer's nanites.
The Doctor and Evelyn have stayed in Appleton for a few days after dropping off Brewster, to watch on him.
The Doctor picks a lock with a hairpin and is lending his engineering expertise to Mr Stretton.
Links: The Feast of Axos (Evelyn mentions talking the Doctor into giving Brewster a second chance), The Mark of the Rani ("before you can say Miasimia Goria"). Evelyn refers to "Spare Parts" (possibly a red herring designed to suggest the involvement of Cybermen in the story.)
Untelevised Adventures: The Doctor mentions Venice "You should have seen it in the good old days."
Location: Appleton, Lancashire during the Industrial Revolution, perhaps in the 1860s (the Doctor says copper extrusion, developed in 1894 by Alexander Dick, will not be realised for another thirty years.)
The Bottom Line: "Left to their own devices, you could say?"
Some common elements of Robson's work, particularly that of I.D. (out of control, sentient machinery) reappear, and the guest cast are fun, from the sardonic Townsend, the pompous Stratton to Rory Kinnear's shifty but ineffectual Belfrage. If anything, Evelyn gets the short straw among these types, and if you're no fan of Brewster then the ending may not be the one you're looking for either.