In "The Age of Steel," the Doctor gets to revisit one of his key arguments as an experienced traveler; he makes a passionate defense of imagination and emotion as a motivating factor to overthrow all tyranny. It's an argument he has rehearsed many times, against many uncaring foes, and despite the fact that they never listen, he always wins.

He also gets to use emotion as a weapon against his most implacable foes, while sending up the key obsession of the last decade in technology, namely the constant need to upgrade.

Here are a few other things that you should keep an eye out for, the next time you watch.

(The episode is available on iTunes and Amazon.)

The design of the new Cybermen was, according to director Graeme Harper, inspired by Art Deco. This was a deliberate ploy to separate the feeling of the parallel Earth from our reality. So while Russell T Davies wanted his Cybermen to be men of steel, rather than the silver warriors they had been in the past, the cut of their armor was inspired by Art Deco sci-fi landmarks such as Fritz Lang's Metropolis, and Doctor Who's own robotic designs, particularly the giant K1 robot in "Robot" and the eponymous villains in the Fourth Doctor adventure "The Robots of Death."

When cross-examining his Cybermen to find out what they think about, Lumic expresses his approval of their homicidal tendencies with a suitably despotic "excellent," which is something of a catchphrase of the Cybermen from the Doctor's parallel dimension, as this fan-made montage proves:

https://www.youtube.com/watch?v=Mt67k3AEl1s

Some of the shots shown of the city of London from above were taken from unused footage filmed for the Ninth Doctor two-part story "Aliens Of London" / "World War Three."

The Doctor refers to their three-point attack on the Cybus factory as being from "above, between, below," which is a reference to the 1983 20th anniversary Doctor Who special "The Five Doctors." The Second Doctor refers to a Gallifreyan nursery rhyme about the three entrances to the Tomb of Rassilon.

Similarly, when the Doctor realizes he can switch off the Cybermen's emotional inhibitors, he pauses to ask himself, "Do I have that right?" This is the same line the Fourth Doctor used when pondering whether he could wipe out the earliest form of his most deadly foes in "Genesis of the Daleks."

https://www.youtube.com/watch?v=9PXdwqlJ19U

Whether by accident or design, the Doctor's description of the people reporting to Battersea Power Station as sheep—"Give them their minds back, so they don’t walk into that place like sheep"—has strong echoes (no pun intended) with the work of Pink Floyd. The song "Sheep," from their 1976 album Animals, contains a particularly prescient lyric—"Meek and obedient you follow the leader / Down well trodden corridors into the valley of steel"—which could count as merely coincidental had that album not also featured Battersea Power Station (complete with flying inflatable pig, not unlike Lumic's airship) on the cover. For their part, the Floyd had been known to throw a reference to the Doctor Who theme into their live performances of "One of These Days."

During the battle Mickey and Jake have with their reanimated Cyberman, Mickey goads him into throwing a punch with the time-honored British taunt "Come and have a go," only leaving off the second half of the sentence, "if you think you're hard enough." This line, well known to British viewers, has been shouted across pub car parks and city high streets, and even gets chanted at football matches.

Originally, the end of the story featured Lumic the Cybercontroller ripping away the floor of an elevator with the Doctor and Rose inside, rather than climbing a rope ladder. And once he'd been dispatched, the Doctor went off to fetch Jackie Tyler from Rose's world, reuniting her with Pete. But this would mean Rose saying goodbye to her mother and choosing to continue life with the Doctor, an even more wrenching farewell than her departure from Mickey, so, as there was a plan for this alternate reality to make a repeat appearance at the end of the season, Russell T Davies opted to separate Jackie and Pete for the time being.

https://www.youtube.com/watch?v=c_mH1ERYk0k

The Cyberman who identifies herself as Sally, and whose emotional inhibitor is first turned off, was originally called Kerry. Her demise was supposed to occur earlier in the script, in front of not just the Doctor and Mrs. Moore, but also Rose, Mickey, Pete and Jake. In the event, putting Sally's last moments later on allowed Mrs. Moore a bigger role in proceedings, as she had originally be earmarked to perish in the cooling tunnels.

And one final change in the edit: It was originally scripted that Jake would confirm he and Rickey were a couple, during the section where Mickey pledges his support and that he wouldn't try to take his doppelganger's place. It was felt that Jake's reaction worked without having to explicitly label the relationship he had with Rickey, so the line was cut.

NEXT: 10 Things You May Not Know About ‘The Idiot’s Lantern’

Now read the rest of the 10 Things You May Not Know About Doctor Who archive.