"The Stolen Earth" is not just the story that brings together all of the underlying themes of the first four seasons of Doctor Who, it's also a chance to evaluate the character of the new Doctor, bringing back a character from his past who can see the devastating effect he has on the people who fall into his wake.

It is also a story — coupled with "Journey's End" — that seriously challenges fan-accepted knowledge around the Doctor and his ability to regenerate, and brings Davros back, showing the opposing side of the Time War. All this plus some top quality references to the past and the return of the great Harriet Jones catchphrase.

Here are a few things to keep an eye out for, the next time you watch:

(The episode is available on Amazon and iTunes.)

Before Freema Agyeman had even been cast as Martha Jones, Russell T Davies planned that this story would bring together all the key elements of the first four seasons of the show, including the two TV spin-offs he had launched since Doctor Who returned to TV; Torchwood and The Sarah Jane Adventures. In his book The Writer's Tale, he includes a message to the production team which explains the scope of his ambition:

"The season finale. Earth is transported halfway across the universe as part of a Dalek plot. These episodes feature Martha, Captain Jack, Sarah Jane, Elton, and Rose. Jackie and Mickey? Also, can I have the Torchwood team, just for a couple of days? Plus, a futuristic space station complex where lots of alien races are gathering for a conference. CGI: Bane, Krillitanes, Gelth, Isolus, everything we've got in the computer.

"Lots of gunfire and exterminations. And the biggest Dalek spaceship interior ever – more like a Dalek Temple. Christ almighty! The skies over the Earth need to be changed to weird outer space vistas. Also, visible in the sky, a huge Dalek ship interior. The size of a solar system! This will probably explode. Like they do. And Davros."

He also planned to include Alonzo Frame and Donna Noble, making cameo appearances. At the time of creating his big idea, they hadn't secured Catherine Tate's services for Season Four and Russell had a different companion in mind. Alonzo Frame, played by Russell Tovey, was intended to become part of the Shadow Proclamation, in some early drafts of the script.

The Shadow Proclamation was originally conceived as a full meeting of known alien races that included: Raxacoricofallapatorians (with the young Margaret, who had been returned to childhood by the Doctor in "Boom Town"), Adipose (both baby and adult), Krillitane, Graske, the Moxx of Balhoon, Sisters of the Wicker Place Mat, Vespiform, Hath, Cybermen, Sycorax, Isolus, Graske, Krotons and Hoix (last seen at the beginning of "Love and Monsters"). It was considered too expensive, so the Judoon were brought back instead.

Harriet Jones's subwave network was created by the Mister Copper Foundation. Although not made explicit in the script, as far as Russell T Davies is concerned, this was the organization founded by Copper, the so-called professor of Earthonomics from "Voyage of the Damned" who arrived on Earth with a fortune in a credit card.

https://www.youtube.com/watch?v=Lf1VvuhLDHs

There are a couple of noteworthy crossovers with Douglas Adams in the script. First, there's the whole idea that some bees might be an alien species smart enough to leave the planet once they'd realised it was about to be under attack, which echoes the actions of the dolphins in The Hitchhiker's Guide to the Galaxy. There's also the guest appearance by Richard Dawkins, who was at the time married to Lalla Ward, who played the Second Romana, companion to the Fourth Doctor. The couple were first introduced by Douglas Adams, who was Doctor Who script editor during Lalla Ward's time on the show. There's a suggestion that the Dalek saying "resistance is useless" is a nod to the same line being used by the Vogons in Hitchhikers, but that line goes all the way back to the First Doctor epic "The Dalek Invasion of Earth", so if there's a crossover there, it went in the other direction.

The Daleks are clearly fond of using "the Abomination" as a nickname. Not only is it their pet name for Dalek Caan, it was also the name the Dalek Emporer gives to Rose once she has become the Bad Wolf in "The Parting of the Ways."

Among the real TV people to appear, showing media reactions to the changing sky, was British chat show host Paul O'Grady, fulfilling Season Four's guest comedian quota. Paul originally made his name portraying the scathing drag character Lily Savage on British TV, and even featured this Doctor Who skit in his 1997 BBC sketch show:

https://www.youtube.com/watch?v=YEm7LOW5ktY

One of the planned scenes involved a Dalek spaceship landing in Westminster, by the Houses of Parliament, and then the extermination of the Prime Minister, who was named Aubrey Fairchild. As this element of the plot was excised in favor of the return of Harriet Jones, former Prime Minister (you know who she is), the name Aubrey Fairchild was then used as part of the character list for the Christmas special "The Next Doctor."

The Doctor makes reference to someone attempting to move the Earth "a long time ago," which is a throwback to the First Doctor adventure "The Dalek Invasion of Earth," in which the Skaronian despots attempted to convert the planet into a colossal spaceship by inserting a drive into the planet's core. Don't get too comfortable though, according to events in the Sixth Doctor season-long epic "Trial of a Time Lord," Earth will be moved two light years away and renamed Ravolox in the year 2,000,000.

While it should come as no surprise to find out that the Doctor's phone number — 07700 900461 — doesn't really exist, it does follow a similar pattern to real phone numbers that were issued to cellphone users in Jersey. In fact, it's one of a series of phone numbers created by the media and telephone regulatory body OfCom to be used within dramas. If a drama shows a dialled cellphone number and it begins 07700 9, then it's one of those. And of course, there's no problem if two dramas use the same number, because they're all fake anyway.

NEXT: 10 Things You May Not Know About ‘Journey’s End’

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