Fans have always wondered at what exactly may be inside the multi-dimensional maze in a hall of mirrors that is the interior of the TARDIS. The Doctor has offered clues down the years—a swimming pool, bedrooms, lots of wardrobe space—but a true journey into the ship's dark recesses doesn't happen very often.

"Journey to the Centre of the TARDIS" comes largely from Steven Moffat's wish to make that trip, to have a good poke around in the Doctor's corridors and see what is what.

Here are a few things to keep an eye out for, the next time you watch:

(The episode is available on iTunes and Amazon.)

Stephen Thompson, scriptwriter, told Doctor Who Magazine that his brief from Steven Moffat was twofold, partly to indulge his inner geek ("I’m a pure mathematician and anything involving multi-dimensional geometry gets me excited") and partly to finish the idea started in the Fourth Doctor story "The Invasion of Time." That story was supposed to explore the inside of the TARDIS, but industrial action by TV workers meant that there was a rush to create the sets: "Studio sets were not built, and as a result our only glimpse of the TARDIS interior has been a disused hospital in Surrey with bin-bags stuck to the windows."

Unlike a lot of Doctor Who scripts, there are no working titles for this story, as "Journey To The Centre Of The TARDIS" is a nod to the Jules Verne science fiction classic A Journey To The Centre Of The Earth, first published in 1864 (as Voyage au centre de la Terre).

Two firsts exist in this story: This is the first televised Doctor Who adventure to use the word TARDIS in the title, and it's also the first story in the relaunched series in which the Doctor closes the TARDIS door from the console. This was something that happened very frequently in the classic series, when the TARDIS doors were emphatically not police box doors on the inside.

One of Stephen's original ideas was that the TARDIS would somehow collide with a school trip, and become overrun by teenagers. This was partly inspired by his own experiences as a maths teacher.

Other abandoned ideas included Gregor van Baalen being a cyborg, or at least the beneficiary of various robotic enhancements, and their salvage business having been set up to deal with a severe metal famine. His brother Tricky (short for "Electricky") was originally named Sander. And there was a scene in which Clara would enter a room containing items that had belonged to all of the Doctor's previous companions. This was later reduced to young Amy Pond's homemade model TARDIS, a magnifying glass and umbrella (that resembles one used by the Seventh Doctor) and the Doctor's cot.

The music the van Baalen brothers put on while suiting up to employ the magno-grab (and therefore the song that the Doctor uses his sonic screwdriver to seek out in order to find his way back to the moment the TARDIS is attacked) is "Fire Woman" by The Cult:

https://www.youtube.com/watch?v=8g6h1vI4Xv0

As Bram begins to strip the console of the TARDIS, sound clips can be heard of various moments from Doctor Who's past. These are:

Susan Foreman saying, "I made up the name 'TARDIS' from the initials: Time and Relative Dimension In Space" from "An Unearthly Child"

The Third Doctor saying, "The TARDIS is dimensionally transcendental" and Jo Grant replying "What does that mean?" from "Colony in Space"

The Eleventh Doctor saying, "You sexy thing," Idris asking, "See, you do call me that! Is it my name?" and the Doctor answering "You bet it's your name!" from "The Doctor's Wife"

The Fourth Doctor saying, "That's trans-dimensional engineering. A key Time Lord discovery" from "The Robots of Death"

The Ninth Doctor saying, "The assembled hordes of Genghis Khan couldn't get through that door, and believe me they've tried" from "Rose"

Martha Jones saying, "It's just a box with that room crammed in!" from "Smith and Jones"

Amy Pond saying, "We're in space!" from "The Beast Below"

Ian Chesterton asking, "A thing that looks like a police box, standing in a junkyard, it can move anywhere in time and space?" from "An Unearthly Child"

The Fifth Doctor asking, "You've changed the desktop theme, haven't you?" from "Time Crash"

And as the Encyclopaedia Gallifreya leaks information, the voice of Timothy Dalton as Rassilon can be heard, taken from "The End of Time."

Mark Oliver, who played Bram van Baalen, already knew Matt Smith before filming commenced, as the two had appeared together in the BBC’s 2009 police thriller, Moses Jones.

Ashley Walters, who plays Gregor van Baalen, will have been a familiar face for music loving British viewers from his former life as Asher D, rapper in the UK garage collective So Solid Crew:

https://www.youtube.com/watch?v=g7VhofoV3qs

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