"The Empty Child" is one of the most celebrated, iconic and original stories in Doctor Who. It has the distinction of changing the way people behave around an inanimate object—the World War II gas mask—and takes full advantage of the potential of Doctor Who to tell a story that very few other science fiction realities could manage.

It is also the story that sees young Steven Moffat take up the challenge to write for the Doctor and welcomes John Barrowman into the family too.

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.)

Although this story marks the first appearance of Captain Jack Harkness, his name was originally intended to be Captain Jax (in fact the second part of this story had the working title "Captain Jax"), and he would have been a second companion to the Doctor, alongside Rose Tyler. Jack Harkness was only intended to be his cover name for this story, as he tracked a child-like creature across wartime London. These details, which appeared in Russell T Davies' pitch document for the rebooted Doctor Who, were amended at the scripting stage.

Other potential plot points that were abandoned included the appearance of the boy Jamie's father, a shadowy and silent figure who helped Nancy and the urchin gang find good places to steal food. The reason for his silence and the need to remain hidden becomes clear when it is revealed that he is German. Also, the nanogenes were originally given the name "nanites," but with all the Spock references in the script already, it was felt that there would be confusion as there are also nanites in the Star Trek universe.

Although created with the working title of "World War II", some of the script's early drafts were also headed "An Empty Child," a tip of the cap to the very first Doctor Who episode, "An Unearthly Child." The TV listings for this episode (and the DVD description) also read "London is being terrorized by an unearthly child."

Jack is described as being a former Time Agent from the 51st century. This is one of those subtle references to classic Doctor Who on which the plot does not hinge, but in the Fourth Doctor adventure "The Talons of Weng-Chiang," there's a bad guy named Magnus Greel who is a time traveler from the 51st century, and mistakes the Doctor for a pursuing Time Agent. The Time Agents, while never a firm fixture of the TV show, have made more than a few appearances in Doctor Who spin-off novels (notably Emotional Chemistry by Simon A. Forward, Eater of Wasps by Trevor Baxendale and Trading Futures by Lance Parkin).

When the Doctor meets Nancy and her gang of urchins, one of them asks if he's a "copper," meaning a policeman. In case you're unaware of this British slang, there are several theories as to how this word evolved, including acronyms for cop (constable on patrol) and copper badges on uniforms, but the most likely explanation is that copper derived from the British slang term cop, meaning to catch. A copper was literally someone who caught criminals. The American term cop (which only travelled back to the U.K. with American movies and TV shows) most likely traveled across the Atlantic with Irish immigrants in the last 19th and early 20th centuries.

https://www.youtube.com/watch?v=7iAh4gSsVjo

In the scene where Dr. Constantine (played by Richard Wilson) transforms, his face becoming the gas mask, there was one sound effect that was considered, but cut for broadcast. It implied that his skull was cracking under the transformation, and as producer Phil Collinson told BBC News in 2005, it sounded all too real: "The whole sound effect that went with that was a lot more visceral. We watched it for the first time and said that was crossing over the line because it was a bit too horrible."

Steven Moffatt threw in a knowing wink to W. E. Johns and his stories of the daring pilot Biggles by giving the name Algy to the man who reprimanded Captain Jack for ogling Rose Tyler. In the Biggles stories, Algernon "Algy" Lacey accompanies his cousin James Bigglesworth on various intelligence-gathering adventures, taking mechanic Flight Sergeant Smyth and young Ginger Hebblethwaite along for the ride.

https://www.youtube.com/watch?v=K91dLbiHR14

In the back-and-forth between the Doctor and Rose when he realizes she has renamed him Spock, there's a line that was cut. Rose says "Don't you ever get tired of Doctor? Doctor who?" and the Doctor originally replied "I'd rather 'Doctor who?' than Star Trek."

Captain Jack boasts of having a Chula warship, but the name Chula comes from a restaurant in Hammersmith, London where the writers of the first season of the reborn Doctor Who—Mark Gatiss, Paul Cornell, Robert Shearman, and Steven Moffat—went out to celebrate their good fortune and talk over ideas in February 2004.

Speaking of Steven Moffat and good fortune, "The Empty Child" and its sequel "The Doctor Dances" won the 2006 Hugo Award for Best Dramatic Presentation, Short Form. Not bad for a Doctor Who beginner!

NEXT: 10 Things You May Not Know About ‘The Doctor Dances’

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