The TARDIS seems to have materialised near the modern-day English town of Devesham, but something is very wrong. People in hazard suits attack with guns in their fingers, the pub calendar repeats the same day again and again, every coin is minted from the same date. Even the Doctor and Sarah's friends from UNIT behave unnaturally. And, perhaps most strangely, astronaut Guy Crayford is in charge of the nearby Space Defence Station... but Crayford was lost in deep space years before. The time travellers soon realise that they are not on Earth at all, but in a simulacrum created by the Kraals as part of a plan to invade the Earth.

Although Terry Nation had written or co-written ten Doctor Who stories during the programme's first twelve seasons, only 1964's The Keys Of Marinus had omitted his most famous creations: the Daleks. Latterly, Nation had had an informal agreement to provide one Dalek serial per season, the most recent of which had been Genesis Of The Daleks. When it came time to discuss Nation's contribution to Season Thirteen, however, he was made aware that the new production team of producer Philip Hinchcliffe and script editor Robert Holmes preferred to develop original monsters, rather than recycling old foes.

As such, Nation developed two non-Dalek storylines. The first, entitled “The Enemy Within”, was commissioned on November 29th, 1974. It was followed by “Return To Suknan” on February 13th, 1975. While the latter appears to have been abandoned, Nation was contracted to provide full scripts for “The Enemy Within” -- now under the title “The Kraals” -- on February 27th. This narrative was inspired by fears that the Soviet Union had constructed detailed replicas of Western towns in remote areas, as training grounds for sleeper agents.

In an abandoned plot element, it was realised that the androids were mirror images of the people they were imitating

Nation envisaged the Kraals as being somewhat insectoid in appearance, although this idea was not reflected in the design work. Also abandoned was a key plot element in which it was realised that the androids at the heart of “The Kraals” were not duplicates of the people they were imitating, but rather their mirror images. Indeed, this was how the Doctor deduced that “Sarah Jane” was actually a robot in Episode Two. However, it was eventually decided that this effect would be too technically demanding to realise, and the action was suitably amended.

Nation delivered his draft scripts for “The Kraals” around the start of May. Later the same month, Hinchcliffe advised him that the number of sets would have to be reduced, as would the amount of material to be captured on film. Nation agreed that Holmes should make any necessary alterations. It was during this process that the adventure's title reverted to “The Enemy Within” and then, on July 14th, became The Android Invasion.

An important change to be addressed by Holmes was the elimination of Brigadier Lethbridge-Stewart from the narrative. Although The Android Invasion would be one of the increasingly-rare Doctor Who serials to feature the United Nations Intelligence Taskforce (UNIT), actor Nicholas Courtney advised the production team that his commitments to a theatrical tour would preclude his involvement. Holmes replaced Lethbridge-Stewart with a new character called Faraday; he was conceived as a brigadier as well, but was later demoted to colonel.

A familiar face was assigned to direct The Android Invasion: Hinchcliffe's predecessor, Barry Letts. When he left Doctor Who, Letts had intended to continue as a producer on other projects, but these had all become mired in BBC politics. Instead, he secured permission to work out the remainder of his contract as a director. Letts' last such credit on Doctor Who had come on the Third Doctor's final adventure, 1974's Planet Of The Spiders.

Letts began work on The Android Invasion with five days on location in Oxfordshire. July 21st was spent at the Harwell Atomic Research Establishment, on the grounds of the National Radiological Protection Board in Harwell. The brand-new facility served as the Space Defence Station. The next two days were devoted to scenes in the woods around the faux Devesham. The principal location was Tubney Wood in Tubney, although the morning of the 22nd was spent at Worsham Quarry in Witney. During filming at Tubney Wood, Tom Baker submerged himself in a pond and swallowed so much brackish water that he had to be taken to a hospital to have his stomach pumped.

Letts' team briefly returned to Tubney Wood on July 24th, but this day and the next were largely spent in the village of East Hagbourne, which served as “Devesham”. When writing the Episode Three scene where the Doctor was held captive in the centre of the village, Nation had been uncertain what kind of structure would be available to which the Doctor could be secured. He had assumed it would be smaller than the memorial cross ultimately used and so, in the script, Sarah rescued the Doctor simply by slipping the bindings over the top of the structure. The use of the sonic screwdriver was a late amendment to address the practicalities of the scene as filmed.

Rejoining Doctor Who for the studio recordings were Ian Marter as Harry Sullivan and John Levene as RSM Benton. Marter had just wrapped up his regular role on the series in April, at the end of the previous production block. It had been planned that Harry would continue to be seen in Doctor Who on occasion, but The Android Invasion would prove to be his final appearance. For Levene, this serial marked the last in a series of regular appearances dating back to 1968. Neither Marter nor Levene found The Android Invasion a pleasant experience, as the new production team's deemphasis of the UNIT format was now very apparent. Levene also missed Courtney's presence on set.

As usual, The Android Invasion was recorded in fortnightly two-day studio sessions, held on Mondays and Tuesdays. The first block took place on August 11th and 12th at BBC Television Centre Studio 3 in White City, London. The Monday was principally concerned with the majority of Episode One, alongside scenes in the Kraal cell and corridor for Episode Three. Most of Episode Two was completed on the Tuesday, as was material in the disorientation chamber and the adjacent corridor for the third installment.

Barry Letts ran out of time to record a sequence which explained how the Doctor reactivated his android duplicate

For the second block, on August 25th and 26th, the venue shifted to TC8. The remainder of Episode Three was largely taped on the Monday, together with Episode Two sequences in Styggron's control room and the Defence Centre corridor. This left Episode Four to the Tuesday, plus Episode Three material in the loading bay and an ultimately unused scene in the scanner room for Episode One. Unfortunately, Letts ran out of time on this day, resulting in the loss of a sequence from late in the final installment. It would have explained how the Doctor reactivated his android duplicate, as well as accounting for the fate of the Kraal invasion armada.

On December 13th, the concluding episode of The Android Invasion was scheduled for 5.55pm, ten minutes later than usual. This was because Grandstand, broadcast earlier in the day, was slightly extended to accommodate the FA Cup Draw. Doctor Who then went on a two-week hiatus over the Christmas period before returning in the new year with The Brain Of Morbius.

• Doctor Who Magazine #193, 25th November 1992, “Archive: The Android Invasion” by Andrew Pixley, Marvel Comics UK Ltd.
• Doctor Who Magazine Special Edition #8, 1st September 2004, “Bohemian Rhapsody” by Andrew Pixley, Panini Publishing Ltd.
• Doctor Who: The Complete History #24, 2016, “Story 83: The Android Invasion”, edited by Mark Wright, Hachette Partworks Ltd.
• Doctor Who: The Handbook: The Fourth Doctor by David J Howe, Mark Stammers and Stephen James Walker (1992), Virgin Publishing.
• Doctor Who: The Seventies by David J Howe, Mark Stammers and Stephen James Walker (1994), Virgin Publishing.
• In·Vision #10, November 1988, “Production” edited by Justin Richards and Peter Anghelides, Cybermark Services.