"New Earth" is a story that has at its heart a very British view of class warfare. There are rich, entitled people with diseases whose lives must be saved, and an underclass who have been given diseases on purpose in order to save them. Along the way, there's the language of snobs, a righteous insurrection and a comedic and flirtatious triangle between Rose Tyler, the Doctor and an out-of-body Cassandra. Oh, and a lot of cats in wimples.

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.)

Working ideas for what became "New Earth" included setting it on a planet called Coffra, in a facility called the Hospital of Evergreen Days and calling the feline medical staff the Sisters of Patience. Chip, Cassandra's servant, was conceived as a comic aside, a little person called Zaggit, but as his role in proceedings became more pronounced, he became a more rounded character.

Russell T Davies also said he wrote the episode's Cassandra body-swapping plot with one specific reason in mind, to show off Billie Piper's comic talents. He told the Radio Times: "I promised Billie an episode in which she'd be funny. So episode one of the new series is very much based around comedy for Billie."

In the audio commentary for this episode, David Tennant tells the inside story of Billie Piper's personal transformation into Cassandra. It's all to do with support, apparently. She wore a Wonderbra and a slightly different shade of lipstick for her Cassandra scenes, giving her a slightly different physical appearance to aid the haughtiness and strutting.

https://www.youtube.com/watch?v=8KrmIZhOEZ0

While this is the episode in which the Doctor first says his trademark "I'm sorry, I'm so sorry," it is also notable for another, less well-celebrated running theme. The Tenth Doctor likes little shops, especially in public buildings like hospitals: "A shop does some people the world of good. Not me. Other people..."

The hospital interior shots were filmed in the Wales Millennium Centre, and when the Doctor talks about where the hospital should put their shop, David Tennant points over to the centre's actual shop, which sells Portmerion china. It is off-camera, of course. He returns to his theme in "Smith and Jones" and "Silence in the Library." In the former, as the Judoon invade Martha Jones's hospital, the Doctor pauses to note, "Oh, look down there, you've got a little shop. I like a little shop."

And in the latter, it even becomes a plot point, as Donna points out that public buildings are arranged in a certain way:

Donna: "Doctor, the little shop. They always make you go through the little shop on the way out so they can sell you stuff."

Doctor: "You're right. Brilliant! That's why I like the little shop."

When Cassandra first sees herself in Rose's body, she wails, "Oh my God! I'm a chav!" This slang term is thought to have originally been derived from the Romani word chavi, meaning child. First coined in the late 1800s, the 21st century saw a rebirth of the word as a pejorative term used by the upper classes to describe what they considered to be typical working class behavior (drinking, violence, insubordination and wearing knock-off designer clothes). Tip for travelers to the U.K.: It's the kind of word that starts fights, so be careful who you use it with.

Similarly, when she's informed that Rose speaks "old Earth cockney," Cassandra welcomes the Doctor with "wotcha"—an informal working class greeting (albeit slightly archaic) derived from "what cheer"—and embarks on rhyming slang, including "apples and pairs," meaning stairs, "I can't Adam and Eve it," meaning "I can't believe it," and "boat race," meaning face. The point being, she's got no idea how other people speak and has no inclination to learn.

Russell T Davies has said that he originally intended for the Doctor to alleviate the suffering of the diseased people from the lower floors with a kind of group euthanasia. However, a wry comment from Steven Moffat changed his mind. Moffat wrote a funny line in his introduction to the book Doctor Who: The Shooting Scripts, in which he said Russell "creates interesting characters and then melts them." Challenge accepted: Russell allowed the Doctor to save the day.

https://www.youtube.com/watch?v=JUj6psqDiAE

Adjoa Andoh plays Sister Jatt, one of the cat/nun hybrid Sisters of Plenitude. Due to the extensive hair and makeup required for the role, it was perfectly fine for her to make a return trip to Doctor Who in Season 3 and 4 as Francine Jones, mother of Martha.

Fittingly for the first story of the second season of the new Doctor Who, some of the footage of the lift that Rose rides to the basement is taken from shots originally filmed for the first story of the first season of the new Doctor Who, i.e. "Rose."

And finally, there's an ontological paradox (a loop in time) at the heart of this story, in that Cassandra uses Chip—the man who said she was beautiful at a party and then died—as a model for the clone who eventually became her sole companion. This clone was, as we know, Chip. So she used Chip to make a clone of Chip, leaving us with the question... where did Chip come from in the first place?

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