It is safe to say that most of the attention on this two-part story tends to be lavished on the latter end of things, as events conspire to split the Doctor from Rose, the girl who helped him get over his experiences in the Time War, forever. But there are other things going on at the beginning of this story that are worth a second look, not least the arrival of the Doctor's next best friend (or her cousin, at least), and the return of two of his worst enemies.

So, as we'll be looking at "Doomsday" (and its ending) next week, 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.)

This is the first story in which the Daleks and the Cybermen actually meet. There had been an attempt to write a story that pitted the rotters of Skaro against the metallic men from Mondas as far back as 1967, but Dalek creator Terry Nation vetoed it (the story eventually became the Cyberman epic "The Wheel in Space," which does at least feature some footage from "Evil of the Daleks"). The closest the two monstrous armies ever came to meeting was in "The Five Doctors," which did feature both Daleks and Cybermen, but not face to face.

Having written this two-part story under the umbrella title of "Army of Ghosts", Russell T Davies considered calling the two episodes "Torchwood Rises" and "Torchwood Falls," to pick up on all the various Torchwood references scattered across Season Two. Having already seeded the idea that Rose might die in battle, and with the title "Army of Ghosts" still available, it became clear that "Doomsday" would be a more thrilling title to frame events.

The Cybermen have a real thing about breaking through sheets of plastic. They first did it in the Second Doctor adventure "The Tomb of the Cybermen" (see below) then again shortly afterwards in "The Invasion," and in the Fifth Story story "Earthshock."

https://www.youtube.com/watch?v=zIIKGc6jL8Y

The plan was originally to base this story in Cardiff, around the time rift which featured in "The Unquiet Dead" / "Boom Town," but as Torchwood had been commissioned as a spin-off series by the time this story was preparing to shoot, which also used the time rift as part of its story arc, Russell T Davies chose a familiar London landmark as his new setting, Canary Wharf. Speaking of which...

The helicopter shots of Canary Wharf are taken from the opening credits to the British edition of The Apprentice.

Younger viewers may not know the identity of the gray-haired man who slightly defensively says, "Well no one needs me any more" as the Doctor channel surfs, researching the ghosts. The joke there is that the man in question is British psychic Derek Acorah, the former presenter of Most Haunted, a program in which he (and non-psychic presenter Yvette Fielding) would investigate various buildings that were supposed to be haunted. The show was also the subject of a spoof on Saturday Night Live:

https://www.youtube.com/watch?v=rdJ-6qJbHpw

Other notable clips from British TV include the British chat show Trisha, a fake TV show called Ghostwatch (which shares a name with a controversial 1992 BBC mock-documentary that fooled viewers into thinking it was a live paranormal broadcast) and a scene from the soap EastEnders, in which Barbara Windsor castigates a ghost for being the reincarnation of Denn Watts. The joke there is that Denn Watts (played by Leslie Grantham) had already come back from the dead in EastEnders, having been presumed dead in 1989, and then he was killed again in 2003, having made a shock reappearance.

The Magna-Clamps are said to have been found in a crashed spaceship at the base of Mount Snowden. This will become significant in a season or two, as it was also the site of the Immortality Gate left by the Vinvocci in "The End of Time," the one that brought back John Saxon. So presumably those Magna-Clamps were from the same ship.

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

This story was filmed during the secret search for a new companion, and the production team felt that Freema Agyeman had done a particularly good job playing Adeola Oshodi, so they invited her to audition for what was, at the time, said to be part of the Torchwood TV show, as Billie Piper's decision to leave Doctor Who had been kept secret. As we now know, she got the part, and this required a reference to appear in later scripts that Martha Jones had a cousin who worked for Torchwood. Russell T Davies has since stated that he would have gladly rewritten this episode so that Adeola survived if he had he known Freema would be so perfect for the role.

As with many of the episodes in this season, "Army of Ghosts" was accompanied by an online TARDISode, which showed a reporter investigating the history of Torchwood. This was not the first idea considered, which was the more poignant shot of the Doctor laying flowers at two graves, which turn out to be those of Jackie and Rose Tyler.

NEXT: 10 Things You May Not Know About ‘Doomsday’

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