Written by Matthew Graham.A solar tsunami sends the TARDIS hurtling towards a 22nd Century factory on Earth, where human Doppelgängers ("Gangers") are used to mine dangerous acid. They use the white clone goo to create avatars, which can safely mine while the actual people are controlling them with their thoughts.The Doctor would much rather have this adventure alone, but Amy and Rory insist on coming along. The factory is dangerous. Dangerous enough, in fact, to dissolve the ground around the TARDIS with acid and cause her to sink into the earth. It also dissolved the Doctor's shoes, so he has to borrow a pair of boots.The second wave of the solar tsunami, however, causes the Gangers to separate from the people they're being controlled by. The clones are now separate, independent people, retaining all of their original memories and horrified by the fact that they're not "real". They can remember every second of their "original's" life and feel every emotion they've ever experienced. The white Flesh rebels against the change, and causes their faces to distort.The Gangers quickly feel threatened enough to start a rebel faction against the real humans. This escalates into an all-out battle, cut off only when it turns out that the Flesh has also made a copy of the Doctor.
Acting for Two: Most of the characters in this episode have a corresponding Ganger, and share scenes with them. Including the Doctor.
Captain Obvious: The Doctor determines that "something corrosive" is flowing through the pipe marked "DANGER: CORROSIVE." And again after the storm causes leaks. To be fair, he did just almost get a face-full of Hollywood Acid.
Doctor: It is too dangerous in here with acid leaks!
Ceiling Cling: Ganger Jennifer pulls one off when stalking human Jennifer.
A possible example-to-be for the next episode: Dicken's sneezing may be being set up as a means to differentiate him from his Ganger.
The Ganger Doctor still wearing the right shoes after the Original stepped into the acid and had to replace them might turn into one, as well. Remember — The Flesh mimics the clothing worn by the original the instant it is scanned, and the Doctor walked out into the acid after that.
Cloning Blues: While creating the Gangers isn't quite cloning science-wise, this trope is otherwise in full effect.
The trailer indicates the Gangers may be the same as, or similar to, the state Lady Cassandra was in. They also seem to be "forced-growth clones" like Chip. The Doctor seems somewhat familiar with the flesh used to create them, suggesting it is in a "primitive state".
Creator Thumbprint: MatthewGraham likes the image of a weathervane foreshadowing something important, characters playing darts, using '70s music in his shows, and casting Marshall Lancaster as a bumbling type of character.
Funny Background Event: During the scene where the humans meet up with their Gangers, behind the Doctor you can see Rory react strongly to Jennifer squeezing his hand too tight.
Genre Savvy: Rory. "I am telling you, when something runs towards you it is never for a nice reason!"
Genre Throwback: Basically, Matthew Graham set out to write a Second Doctor story: small cast, minimum amount of effects (mostly practical and make-up) and a very dark, tense tone. Among the Second Doctor tropes on display: A near-future setting, an isolated base in a remote location, an external threat and the possibility of internal treachery, lots of corridors, a trigger-happy paranoid base commander, and a junior member of staff with Photographic Memory who gets compared favourably to a computer.
The Doctor: I have to get to that cockerel before all hell breaks loose. I never thought I'd get to say that again!
No OSHA Compliance: Partially justified in that military facilities and their contractors are usually exempt from many environmental laws. Not to mention that using the Gangers is a safety procedure. They are considered mere tools by the humans, so if one is destroyed it's no different to them from a pair of rubber gloves getting damaged.
Oop North: The monastery seems to be on an island off the Yorkshire coast.
Percussive Maintenance: A weird example. Ganger!Jennifer at one point emphatically beats her fist against her chest; with each hit she instantly shifts between looking normal and look semi-Ganger.
To Blade Runner: Humanoids, created for work, which become more human, and one of the workers "decommissions" a Ganger just as Deckard "retires" replicants.
To The Thing (1982): The flesh can mimic anyone or anything, and can stretch body parts. Buzzer even refers to a ganger as "you thing".
To Avatar: The workers control clones of themselves from harnesses, and the clones (supposedly) become inactive when not being controlled.
To Full Metal Jacket: One of the workers calls Jennifer "Twinkletoes" for making a mistake.
To Invasion of the Body Snatchers (1956): the early shots of Ganger!Jennifer slowly taking on definition are rather... familiar. There's also the general idea of exact duplicates killing and replacing their originals.
The Gangers' Voldemort-esque appearance seems to be lampshaded when Ganger!Jenny says she'll take care of "the spare one".
Along with Voldemort, the design (and abilities) of the Gangers is likely a shout out to Odo and the other Changelings.
Skintone Sclerae: Inverted — the Gangers' skin resembles the white of a human eye.
Sleep Cute: Rory and Amy wake up on the floor next to each other after the tsunami hits.
Used Future: It is the 22nd century, and the acid mine in the monastery is very dark and battered.
What Measure Is a Non-Human?: The Gangers are designed to be expendable — their purpose is to mine the acid, a horribly dangerous job, without putting humans at risk. Naturally, they aren't happy about this.
The Doctor: We're not talking about an accident that needs to be mopped up. We're talking about sacred life. Is everyone clear on that? Everyone clear? Good.
What the Hell, Hero?: Despite the Doctor starting a peaceful dialogue between the humans and gangers, human Cleaves kills Ganger Buzzer, starting a war between the two factions.