Four episodes into the newly relaunched Doctor Who, and it was time to shake things up a little. "Aliens of London" is the story in which we see the true cost of the Doctor's poor timekeeping; we see how it is for the families and friends of the people he calls companions when he takes them off in the TARDIS; and we see what happens the first time humans properly meet aliens from another planet, in a way that can't be hushed up.

It's also the first story in which we meet Harriet Jones, MP, before the standard response to her name was "yes, we know who you are."

Here are a few things that you should keep an eye out for, the next time you watch. (The episode is available on iTunes and Amazon.)

In a marked break from the previous format of Doctor Who, most of the stories for the new season were intended to take place across just one episode. Nonetheless, Russell T Davies wanted some two-part adventures with a traditional Doctor Who cliffhanger in the middle. He had planned a race from the planet Raxacoricofallapatorius very early on, but used the idea of them being a family (called Slitheen) from the Aubertides from Paul Cornell's Doctor Who: The New Adventures novel "Human Nature" from 1995. Of course, Cornell's novel also provided the inspiration for the Tenth Doctor adventure "Human Nature."

One of Davies' original plot ideas was something similar to the classic British sci-fi serial Quatermass and the Pit, in which the the alien spacecraft that falls into the Thames actually crashes on the ground, leaving a big pit for everyone to peer inside. This was changed once Russell realized what could be achieved with CGI.

When Jackie asks the Doctor "How old are you then, 40? 45?" she's not far from the truth, in temporal terms at least. At the time of broadcast, Christopher Eccleston was 41, one year older than Camille Coduri, who plays Jackie. Billie Piper was 23 but playing a character who was 19. To make matters even more temporally confusing, Rose Tyler was 19 but a year out of sync with her own timeline thanks to her travels with the Doctor, so had she not traveled into the future, she would have been 20 in 2006.

While Rose and the Doctor are trying to get to see more of the alien ship that biffed a hole out of the clock tower that houses Big Ben, you can here one of the people in the crowd say, very clearly, "It's got to be Ken Livingstone, innit?" At the time, Ken Livingstone was the Mayor of London and had introduced new traffic calming measures such as 2003's congestion charge to try to encourage people not to drive in Central London.

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

There was a plan that the missing Prime Minister would be lookalike of Tony Blair, therefore fixing the idea that the Whoniverse was a close copy of our own. However, the actor brought in turned out not to be quite as convincing as the producers would have liked, so they avoided all specific references, apart from one oblique line by Harriet Jones, who refers to her lowly status, saying "I'm hardly one of the babes." This was a reference to the British tabloid term "Blair's babes," used to describe some of the female Labour Party MPs who entered Parliament after the 1997 general election.

Speaking of Prime Ministers, there's a subtle historical gag about David Lloyd George, the minister who in 1915 suggested that the U.K. could benefit from a period of enforced prohibition from drinking alcohol. His most notable quote on the subject was "We are fighting Germany, Austria and drink; and as far as I can see, the greatest of these deadly foes is drink." And given that the Doctor often spits out alcoholic beverages, this adds extra fun to the line "I'm telling you, Lloyd George, he used to drink me under the table."

This is the first time the Doctor has visited Albion Hospital, but it won't be the last. Or rather, it will be chronologically but not in his own timeline. By which I mean he goes to the same hospital some 50 to 60 years before this visit, in 1941, for the events of "The Empty Child"/"The Doctor Dances."

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

"Aliens of London" is the last Doctor Who story to feature footage shot in BBC Television Centre, the building in which a great majority of the classic series was filmed. TVC did feature in the 50th anniversary spoof The Five-ish Doctors Reboot but not within the reality of the show.

It is also one of the last times that the acronym UNIT was used carrying its original meaning, United Nations Intelligence Taskforce. After the broadcast of Season One, Russell T Davies told Doctor Who Magazine that the real United Nations had asked for their name to be removed, and so the organization has been renamed Unified Intelligence Taskforce. The name change appeared for the first time in the Tenth Doctor adventure "The Sontaran Stratagem."

That missing year overshadows everything that happens in the contemporary Earth Doctor Who stories for quite a while. As Rose can't go back on herself to 2005 and alien incursions keep happening on Earth in roughly synchronous order from this point onwards, it actually takes a couple of seasons before Doctor Who gets back in sync with the broadcast year, and some fans believe it doesn't really get sorted until "The End of Time," the Tenth Doctor's final adventure.

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