The Doctor saves a soldier named Journey Blue from the Daleks and returns her to her command ship, the Aristotle. There he discovers that the Galactic Resistance has captured a lone Dalek which has vowed to destroy its own kind. The Doctor agrees to investigate the unprecedented malfunction, so he fetches Clara from an awkward encounter with her new colleague, Danny Pink. Back aboard the Aristotle, Journey and the time travellers are miniaturised and injected into “Rusty”. But as they navigate the casing's lethal defences, the Doctor must also confront the question of whether there can ever truly be a good Dalek.

During 2009, the BBC approached incoming Doctor Who executive producer Steven Moffat about developing story ideas for a series of online games which would feature elements from the programme. The Daleks were an obvious monster to include in The Adventure Games, and Moffat initially proposed a scenario in which the player's avatar became miniaturised and infiltrated a Dalek casing. However, he quickly realised that this was a notion worthy of dramatising for television. In its place, Phil Ford -- who had co-written the same year's The Waters Of Mars -- devised City Of The Daleks. Ultimately the first of The Adventure Games to be made available, it was released on June 5th, 2010.

Moffat did not forget the miniaturisation concept, and he returned to it while he was planning Season Thirty-Four. He saw its potential as a vehicle for re-establishing the menace of the Daleks, especially in opposition to the recently-regenerated Twelfth Doctor. Meanwhile, Ford had been invited to pitch ideas for the new season. He was surprised, but not disappointed, to discover that the narrative he was asked to develop was not one that he had submitted, but rather Moffat's Dalek storyline. Ford immediately recognised that it had its roots in Fantastic Voyage, a 1966 feature film in which a team of specialists was reduced to microscopic size and injected into the bloodstream of a dying scientist. He knew that 1977's The Invisible Enemy had taken the same movie as its inspiration, and so he set out to make the Dalek story distinctly different.

Phil Ford developed his first draft as if it featured the Fourth Doctor

Ford began working on his script during the summer of 2013. At this point, Peter Capaldi had not yet been cast as the Twelfth Doctor, and so the writer had to rely on Moffat's description of the approach that he planned to take with his new leading man. Ford felt that this most closely resembled Tom Baker's performance, and so he developed his first draft as if it featured the Fourth Doctor. Ford was wary of including just a single Dalek in his narrative, for fear that it would appear too similar to 2005's Dalek. As such, he added the element of the Daleks invading the Aristotle; originally, they were defeated when the Doctor took control of the malfunctioning Dalek from within.

One of Moffat's goals for Season Thirty-Four was to round out the character of Clara Oswald. During Season Thirty-Three, she had largely been presented to audiences as a puzzle box, with the Doctor trying to solve the conundrum of the “Impossible Girl”. In the fiftieth-anniversary special, The Day Of The Doctor, she had been given a new job as a teacher at Coal Hill Secondary School. Now Moffat wanted to introduce a love interest in the form of one of her colleagues, Danny Lawrence. Since Ford's script was earmarked as the season's second episode, he was asked to set up this new relationship. His initial approach was to assume that Clara and Danny had been dating for some time, but the production team quickly decided that they wanted to depict the pair's original meeting.

By October, the story was called Into The Dalek. At this stage, Journey and Gretchen were childhood friends who had grown up together on a farm. An early December draft was written by Moffat, in which he focussed on refining the interactions between Clara and Danny, whose surname was now Pink. He also added a brief appearance by Courtney Woods, a Coal Hill pupil whom Gareth Roberts had developed for The Caretaker later in the season. An element of Into The Dalek which became more pronounced in its latter drafts was the new Doctor's animus towards soldiers; his refusal to take Journey aboard the TARDIS at the end of the adventure was only added prior to the script readthrough.

Under director Ben Wheatley, Into The Dalek formed part of Season Thirty-Four's first recording block alongside the premiere episode, Deep Breath. Seven existing Dalek casings were made available for the production. One of the props originally constructed for Season Thirty-Three's Asylum Of The Daleks was heavily modified and distressed to appear as Rusty. Joining the programme's cast as Danny was Samuel Anderson; half a decade earlier, he and Jenna Coleman had spent more than a year together on the soap opera Emmerdale.

On January 25th, 2014, the first scenes filmed for Into The Dalek were those on the grounds of Coal Hill School, which was actually Holton Primary School in Barry. Wheatley and his team then moved indoors to shoot material in the staff room and Danny's classroom; Anderson wound up banging his head against the desk so many times that he required an ice pack. January 29th took cast and crew to Uskmouth Power Station in Newport, for sequences in the Dalek power conduit and around the cranial ledge, along with a number of inserts. On January 30th and 31st, the Newport offices of solar cell manufacturer G24 Power provided space suitable for the nano-scale surgical lab. Then it was back to Holton Primary School on February 1st, with cameras now rolling in the office and the corridor leading to the stationery cupboard.

After a day off on Sunday the 2nd, February 3rd and 4th were spent at Roath Lock Studios in Cardiff. The first day dealt with the voyage via the Rescue One capsule and the visual cortex tunnel entrance. The second day began with Ross' death on the cranial ledge, and then Clara and Journey following the lights in the cortex vault. Meanwhile, Moffat had added a new scene to the script in which Gretchen met the Master, whose identity would be obscured by the nickname “Misty”. The rest of the week, from February 5th to 7th, took cast and crew to MOD St Athan in the Vale of Glamorgan, which posed as various areas of the Aristotle. The first day was dedicated to scenes in the hangar. This work continued to the middle day, when Gretchen's death on the ledge was also taped. Wheatley then moved on to various corridor sequences, which were also the focus on the last day at St Athan.

Almost all of the remaining footage for Into The Dalek was recorded at Roath Lock, starting after the weekend with scenes in the trionic power cell on February 10th. Filming on the 11th involved the tubes which led to the food plant, plus Journey's spaceship. The sets for both the TARDIS and the stationery cupboard were in use on February 12th, when several inserts were also captured. Filming on the 13th began with the climactic material on the cranial ledge and within the cortex vault, before proceeding to the food plant. February 14th dealt with the circuit board maze and the Doctor's confrontation with the Dalek mutant; Wheatley also completed the shots he needed in the TARDIS and the food plant before another weekend off.

On May 23rd, Laura Dos Santos returned to play Gretchen opposite Michelle Gomez's Missy

Various shots were on the itinerary for February 17th: Clara and Journey's ascent up the ladder, action on the Dalek spaceship, more Aristotle corridors, and wire work. Both February 18th and March 28th were then devoted to various inserts, as well as model sequences. Meanwhile, on February 24th, Anderson's involvement in Season Thirty-Four was announced by the BBC. Finally, on May 23rd, Laura Dos Santos returned to play the deceased Gretchen opposite Michelle Gomez's Missy -- the new nickname for Misty -- at Dyffryn Gardens in St Nicholas.

A key edit in post-production came at the very end of Into The Dalek. Set aboard the Dalek flagship, it would have depicted the Command Dalek and some of its troops confronting Rusty. However, the damaged Dalek then chose to self-destruct, taking the entire vessel with it. The deletion of this scene would allow for the possibility of Rusty reappearing in a future story.

Into The Dalek was broadcast on August 30th. In the wake of the extra-length season premiere the week before, Doctor Who now took up its regular timeslot for the first part of the run, at 7.30pm. It was still preceded by Tumble, but The National Lottery: Break The Safe now returned to the Saturday schedule to follow Doctor Who.

• The Doctor Who Companion -- The Twelfth Doctor: Volume One, June 2019, “Into The Dalek” by Andrew Pixley, Panini UK Ltd.
• Doctor Who: The Complete History #76, 2015, “Story 243: Into The Dalek”, edited by John Ainsworth, Hachette Partworks Ltd.