After all of the emotional upheaval of the Doctor's recent adventures, in which he lost Rose, his best friend, and was turned down by Donna Noble, "Smith and Jones" is the moment at which he gets his mojo back. He may start off hidden among humans, investigating an anomaly for something to do, but by the end he's so fully invested in saving the life of his newest acquaintance that he carries her prone body through an oxygen-depleted hospital.

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.)

"Smith and Jones" (working title: "Martha) features quite a cast list of Doctor Who veterans. As well as Freema Agyeman, who had previously played Adeola Oshodi—cousin of Martha Jones—in "Army of Ghosts," there's Adjoa Andoh as Martha's mother, who previously appeared as Sister Jatt in "New Earth." Then there's Anne Reid as Florence Finnegan/The Plasmavore, who appeared in the Seventh Doctor story "The Curse of Fenric," playing Nurse Crane, and Trevor Laird (Clive Jones), who played Frax in the Sixth Doctor story "Mindwarp." Roy Marsden, who played Mr. Stoker, had already lent his voice to the Eighth Doctor audio drama "Human Resources."

In order to try and put a little distance between the characters of Martha and Rose, Russell T Davies toyed with the idea of taking Martha from a point further back in history. She was already slightly older and more knowledgeable than Rose, but there was also a plan for her to meet the Doctor in 1914. This time zone was, of course, revisited by the Doctor and Martha in "Human Nature" / "The Family of Blood."

https://www.youtube.com/watch?v=CwYdpI8UwnY

One plot development that didn't make the final script involved the Doctor and the Plasmavore racing each other to the hospital basement to try to get to the TARDIS first. But Russell T Davies preferred the idea that the Doctor could not access the TARDIS. He also junked the idea of Martha and the Doctor escaping the Judoon by using the window cleaner's cradle, which eventually resurfaced in Donna Noble's first proper companion story, "Partners in Crime."

Other cut scenes include one set in Martha's living room, so that the story didn't jump from the hospital to Leo's party, and one particularly gruesome effect, in which the Judoon's guns would appear to boil the skin off their victims. There was also a need to redo one scene, in which the Doctor is seen using the sonic to enter the X-Ray Room, despite the fact that he had already damaged it beyond repair at that point.

The line "Judoon platoon upon the Moon" is a production in-joke aimed at David Tennant. When speaking the Doctor's lines in an English accent, his natural tendancy towards rounded Scots vowels made it harder to say any word with an "-oon" suffix. Naturally he rose to the challenge admirably.

https://www.youtube.com/watch?v=oNmanlaP1eU

There is some debate over why Russell T Davies chose to name the hospital consultant Mr. Stoker. Given that the principal villain sucks blood from the necks of her victims (albeit using a straw), the production team assumed he had been thinking of Bram Stoker, creator of Dracula, and even wrote "B Stoker" on his office door. However, Russell later said he'd been thinking of a character from the ITV drama Children's Ward, for which he had been both a producer and writer.

While pumping the Plasmavore for information, the Doctor claims to have got a "GCSE in geography," referring to the British exams teenagers take around the age of 16.

The Doctor describes the Plasmavore as being "on the run. Like Ronald Biggs in Rio de Janeiro." This is a reference to the British criminal Ronald (popularly known as Ronnie) Biggs, who took part in a notorious train robbery in 1963. He escaped justice by moving to Brazil, a country with no extradition treaty with the U.K., a similar state of affairs to those that lead the Judoon to corner the Plasmavore by moving her hospital to the "neutral" territory of the moon.

There's an intriguing moment when Martha asks the Doctor if he has a brother and he replies, "Not any more". This is a reference to the Doctor Who: The New Adventures novel Tears of the Oracle, by Justin Richards, in which the Doctor's elder brother appears. His name is Irving Braxiatel. Tears of the Oracle was edited by Simon Winstonek, who also script edited "Smith and Jones."

Martha looks over the Slab that tried to attack the Doctor, asking, "So what is that thing? And where's it from? The planet Zovirax?" which only really works as a joke if you happen to be aware of a series of TV commercials for Zovirax cold sore treatments, in which a young woman tries to hide her facial blemish by going about her daily life wearing a motorbike helmet with a darkened visor:

https://www.youtube.com/watch?v=BBQEzGEhH2Y

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