Aware that his companion is uncomfortable around the TARDIS, the Doctor shifts the time machine into a low-power mode to help Clara get accustomed to her new home. In the process, however, he inadvertently leaves the TARDIS vulnerable to an intergalactic salvage vessel run by the van Baalen brothers, whose attempts at seizure critically damage the interior. With time and space running amok, Clara is trapped in the Ship's expanse while strangely burned monsters prowl the corridors. To rescue her, the Doctor is forced into an uneasy alliance with the van Baalens and their android, Tricky.

Despite his lifelong love of Doctor Who, executive producer Steven Moffat had never forgotten one occasion on which the programme had let him down. This was March 1978, when the Radio Times billing for the final episode of The Invasion Of Time had promised an exploration of the TARDIS interior. The sixteen-year-old Moffat had anticipated a dazzling expedition into the mysteries of the time machine's labyrinthine depths. However, these scenes had been primarily filmed at a disused hospital due to an industrial dispute, and the results were underwhelming. Thirty-four years later, Moffat was the man in charge of Doctor Who, and he decided to seize the opportunity to revisit the unrealised potential of The Invasion Of Time.

One of the writers under consideration for Season Thirty-Three was Steve Thompson, who had written The Curse Of The Black Spot the previous year. Thompson was planning to propose an adventure which would examine the formative years of Taren Capel, the villain behind the 1977 classic The Robots Of Death. Instead, he found himself asked to develop a story which would venture into the heart of the TARDIS. Thompson held a degree in pure mathematics, and Moffat thought that this background might help fuel his ideas. He initially considered setting his narrative in a twisted environment which resulted from a collision between the TARDIS and another time machine, the pilot of which was scheming to steal the Doctor's vessel. Thompson then sought inspiration in his days as a teacher, with the TARDIS instead crashing into a school trip. A gaggle of teenagers would be unleashed into the time machine, and they would cause it to malfunction. Moffat disliked this idea, but he was more interested in Thompson's suggestion that the students could be replaced with a salvage team.

The image of the TARDIS engines frozen in mid-explosion was suggested by Concert For Anarchy, a sculpture by Rebecca Horn

It was Moffat who suggested the episode's title: Journey To The Centre Of The TARDIS, alluding to the classic Jules Verne novel A Journey To The Centre Of The Earth, first published in 1864 as Voyage au centre de la Terre. He also contributed the element of the TARDIS engines being frozen in mid-explosion. The image was suggested by his viewing of Concert For Anarchy (1990), a mechanised sculpture by German artist Rebecca Horn, in which the components of an overturned piano -- suspended from the ceiling -- periodically exploded outwards.

Thompson submitted his first draft of Journey To The Centre Of The TARDIS on June 22nd, 2012. At this stage, the youngest van Baalen brother was named Sander and was not an android, while Gregor was disfigured and had numerous metal attachments. In the TARDIS, Clara found the Doctor's bedroom. It contained the tweed jacket which had been part of his current incarnation's original ensemble, plus photographs of former companions Rose Tyler and Amy Pond.

In mid-July, draft two replaced Sander with the android Tricky -- meant to be short for “Electric-y” -- while the Doctor's bedroom was now a storage chamber which contained all of his former companions' leftover belongings. This was the only version of the narrative which suggested that the Van Baalens' salvage career was motivated by a severe metal famine. Subsequent drafts further simplified the storeroom sequence. Clara now found two artefacts introduced during Season Thirty-Two: the Doctor's cot from A Good Man Goes To War, and a model TARDIS built by the young Amy Pond, as seen in Let's Kill Hitler. The engine room environment only latterly became a ravine, having earlier appeared as a garden.

Journey To The Centre Of The TARDIS was originally intended to be the sixth episode of Doctor Who's Spring 2013 run. As such, it ended with a coda in which Clara's charges, Angie and Artie Maitland, confronted the time travellers about their adventures; this material would lead into episode seven, Nightmare In Silver. Subsequently, Thompson's story was moved to the fifth spot, and the epilogue was rewritten and shifted to the eventual episode six, The Crimson Horror.

Journey To The Centre Of The TARDIS was made on its own as Block Seven of the Season Thirty-Three production calendar, and would be the only Doctor Who installment directed by Mat King. Recording was almost entirely confined to Doctor Who's usual home at Roath Lock Studios in Cardiff. An exception was the early portion of the schedule's first day, September 4th, when the deck of the Hornet was actually a warehouse on Celtic Way in Newport. Later that day, footage of the scoop was captured at Roath Lock; this work continued to the 5th. Corridor scenes were on King's itinerary for the next three days. They were joined by Bram's fall down the shaft on the 6th, material in the cloisters on the 7th, and work on the standing console room set on the 7th and 8th.

Sunday the 9th was a day off, after which recording in the console room resumed on September 10th and 11th. On the latter day, King moved on to scenes in the tunnel beneath the fuel cells, before a brief location excursion took his team to Cardiff Castle in Cardiff, which offered a space suitable for the TARDIS library. The focus on September 12th and 13th was the Eye of Harmony, together with more tunnel sequences on the second day. Material in the Architectural Reconfiguration System and the antechamber to the Eye of Harmony was taped on the 14th, prior to a weekend of rest.

Unfortunately, by the time production on Journey To The Centre Of The TARDIS resumed on September 17th, Jenna-Louise Coleman had fallen ill. As a result, King abandoned his plan to record more console room scenes, in favour of effects and pick-up shots. Coleman returned to Roath Lock on the 18th for material in the tunnel and the storeroom; several inserts were also filmed. Some of the outstanding console room sequences were then completed on September 19th, alongside those in the engine room, while the rest of the footage in the console room was completed on the 24th. Additional inserts were taped on October 18th and November 27th. Then, on November 28th, the mostly studio-bound production ironically ended as it had begun: on location. The venue was the Argoed Isha Quarry near Llansannor, for shots at the ravine.

The Spring 2013 episodes effectively began the countdown to Doctor Who's 50th anniversary

The scene where Bram started to strip down the TARDIS console room offered an opportunity to delve into the Doctor Who back catalogue; this was particularly appropriate, given that the Spring 2013 run of episodes effectively began the countdown to the programme's fiftieth anniversary in November. Audio clips could be heard of Ian Chesterton and Susan from the very first Doctor Who story, 100,000 BC (1963), the Third Doctor and Jo Grant from Colony In Space (1971), the Fourth Doctor from The Robots Of Death (1977), the Ninth Doctor from Rose (2005), Martha Jones from Smith And Jones (2007), Amy Pond from The Beast Below (2010), and the TARDIS itself -- inhabiting the form of Idris -- from The Doctor's Wife (2011).

Journey To The Centre Of The TARDIS aired on April 27th, 2013. It was scheduled at 6.30pm, fifteen minutes earlier than the previous week's Hide. Repeats of Pointless Celebrities' second season now led into Doctor Who, although a news update was still positioned in-between.

• Doctor Who Magazine Special Edition #37, Autumn 2014, “Journey To The Centre Of The TARDIS” by Andrew Pixley, Panini UK Ltd.
• Doctor Who: The Complete History #73, 2016, “Story 236: Journey To The Centre Of The TARDIS”, edited by John Ainsworth, Hachette Partworks Ltd.