The TARDIS seems to have returned the Doctor and Sarah Jane to modern-day England, but it quickly becomes apparent that something is very wrong: the people behave oddly, the calendar has just one day on it, coins are all minted from the same date. The time travellers soon realise that they are not on Earth at all, but on a simulacrum created by the Kraals, who are using the replicated village to help them prepare for their imminent invasion of Earth.

Although Terry Nation had written or cowritten ten Doctor Who stories since 1963, only Season One's The Keys Of Marinus omitted his most famous creations, the Daleks. Latterly, Nation had had an informal agreement with the Doctor Who production team to provide one Dalek serial per season, the most recent of these being Genesis Of The Daleks. When it came time to discuss Nation's contribution to Season Thirteen, however, he was made aware that producer Philip Hinchcliffe and script editor Robert Holmes were far less interested in spotlighting old monsters than their predecessors had been.

Instead, Nation began work on two non-Dalek storylines. The first, entitled “The Enemy Within”, was commissioned on November 29th, 1974, while the other, called “Return To Sukannan”, was requested on February 13th, 1975. The second idea appears to have been abandoned, but on February 27th, Nation was contracted to provide full scripts for “The Enemy Within”, now under the title “The Kraals”.

Originally, a key plot element was that the Kraal androids were in fact mirror images of the people they were imitating, and this was how the Doctor deduced that “Sarah Jane” was actually a robot in episode two. It was eventually decided that this would be too technically demanding to realise, and the action was suitably amended. Corporal Adams was conceived as a regular army private, and the scripts indicated that his first name was Richard. As well, Nation envisaged the Kraals as somewhat insectoid in appearance, although this idea was not used in the design work.

Around the start of July, the adventure's title reverted to “The Enemy Within”; it was classified as Serial 4J. At about the same time, it was learned that Nicholas Courtney would not be available to reprise his role as Brigadier Lethbridge Stewart in the story, because of obligations to a theatrical tour. With the start of production looming, Holmes replaced Lethbridge Stewart with a new character called Faraday, to be played by Patrick Newell. Faraday was conceived as a brigadier, and was later demoted to colonel. On July 14th, the serial's title was finalised as The Android Invasion.

A familiar face was assigned to direct Serial 4J: Hinchcliffe's predecessor as producer, Barry Letts. Letts had been lined up to oversee a biography of Marie Curie, but when this became entangled in internal BBC politics, Letts was given work as a script doctor on various drama programmes. On January 21st, he had been commissioned to write a storyline for Doctor Who entitled “The Prisoner Of Time”, but nothing came of this. Finally, he secured permission to serve out the remainder of his BBC contract in a directorial capacity, and took the reins of The Android Invasion. Letts' last such credit on Doctor Who had been for the final Third Doctor adventure, Planet Of The Spiders, at the end of Season Eleven.

Letts began work on The Android Invasion with five days on location in Oxfordshire. July 21st was spent at the National Radiological Protection Board in Harwell, whose grounds served as those of the Defence Station. The next two days were devoted to scenes in the woods around the false Devesham. The principal location was Tubney Wood in Tubney, although a small amount of work on the 22nd took place at Worsham Quarry in Witney. It was at Tubney Wood that Tom Baker performed a sequence in which he was submerged in a river. Unfortunately, Baker swallowed too much water and had to be taken to a hospital to have his stomach pumped. Finally, material set in “Devesham” itself was filmed in the village of East Hagbourne on July 24th and 25th.

Rejoining Doctor Who for the studio recordings were Ian Marter and John Levene. Marter had just completed his regular engagement as Harry Sullivan in Doctor Who at the end of the previous production block, and was making his lone return to the programme. For Levene, The Android Invasion marked the last of a long series of regular appearances as Benton dating back to 1968. Levene left the acting profession in 1977 and took up a number of different jobs: cruise line entertainer, head of an audio-visual company, even private detective. More recently, Levene moved to the United States and began acting again, including a role in the horror film CanniBallistic! (under his real name, John Anthony Blake). Neither Marter nor Levene found The Android Invasion a pleasant experience, as the new production team's deemphasis of the UNIT format had become progressively apparent. Levene also missed Courtney's presence on set.

The first studio session for The Android Invasion took place on Monday, August 11th and Tuesday the 12th, in BBC Television Centre Studio 3. These days were chiefly concerned with taping the majority of episodes one and two, respectively, although part three scenes in the Kraal cell and corridor were recorded on the 11th, and sequences in the Disorientation Chamber and corridor from the same installment were completed on the 12th.

The second block was scheduled for exactly two weeks later, on August 25th and 26th. This time, the venue was TC8. Again, Letts primarily concentrated on one episode each day, but also performed some shooting for other installments. Part two scenes in Styggron's control room and the corridor of the Defence Centre were taped on the 25th. On the 26th, material in the scanner room from part one and in the loading bay from part three was also recorded. Unfortunately, Letts ran out of time on this day, resulting in the loss of a key scene from late in the final installment which would have explained how the Doctor reactivated his android duplicate, as well as accounting for the Kraal invasion armada.

• Doctor Who: The Handbook: The Fourth Doctor by David J Howe, Mark Stammers and Stephen James Walker (1992), Virgin Publishing, ISBN 0 426 20369 8.
• Doctor Who: The Seventies by David J Howe, Mark Stammers and Stephen James Walker (1994), Virgin Publishing, ISBN 1 85227 444 1.
• 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.
• In-Vision #10, November 1988, “Production” edited by Justin Richards and Peter Anghelides, Cybermark Services.