At Stangmoor Prison, the Doctor and Jo attend a demonstration of the Keller Machine, which purports to remove the negative impulses from the human brain. However, when the Machine is activated, a man somehow dies as if he had suffered his greatest fear. Meanwhile, the Brigadier is responsible for security at the first-ever World Peace Conference, where Captain Chin Lee of the Chinese delegation is conspiring to undermine the negotiations. And, nearby, Mike Yates is preparing to escort a deadly nerve gas missile scheduled for disposal. Connecting all three events is the Master -- the evil mind behind the Keller Machine.

Don Houghton's efficient delivery of Inferno for Season Seven had impressed Doctor Who producer Barry Letts and script editor Terrance Dicks. On June 29th, 1970 he was commissioned to write a treatment for a second serial called “The Pandora Machine” (some sources have suggested such alternative working titles as “Man Hours” and “The Pandora Box” or “The Pandora's Box”). Scripts for the story were requested on August 6th. As with all of the the serials planned for Season Eight, Houghton was asked to incorporate the evil Time Lord called the Master, created by Letts and Dicks, into his storyline.

Houghton was initially inspired by Anthony Burgess' 1962 novel A Clockwork Orange to write a story in which a machine altered criminal minds with the ostensible aim of rehabilitation. He was intrigued by the notion that forcing somebody to be “good” could have sinister implications. However, Houghton was concerned that this idea, on its own, would lack enough substance to sustain six episodes. As such, he adopted a suggestion made his wife, actress Pik-Sen Lim, to add a subplot involving intrigue at an international peace conference.

Jo Grant was originally menaced by her deepest fear, which turned out to be bats

“The Pandora Machine” underwent several alterations during late summer and early autumn. At a preliminary stage, Barnham was killed much earlier in the narrative, and the Brigadier was captured at Stangmoor Prison along with the Doctor and Jo. Under the influence of the Master's hypnotism, Lethbridge-Stewart had the missile's route changed to facilitate its capture by the Master's men. Jo Grant was menaced by her deepest fear, which turned out to be bats. One of the weapons used by the Keller Machine was the image of a Gorgon-like monster, which the Doctor eventually destroyed by showing it its own reflection in the mirror of the missile transport. Houghton originally played up the tension between the United States and China at the conference, with Chin Lee attempting to frame an American delegate for Cheng Teik's murder. An early version of Episode Two's cliffhanger involved Chin Lee trying to blow up the peace conference using an explosive called kredalite.

The Master's pseudonym was originally Emil Dalbiac before the surname was changed to Keller; the first name was also sometimes spelt “Emile”. The Keller Machine itself was first called the Malusyphus box. The female Corporal Bell was initially a male Corporal Bates. Lenny Vosper was named after Houghton's agents, Margery Vosper Ltd, while Cheng Teik was actually the name of his father-in-law. On September 4th, the serial was retitled The Mind Of Evil, a change Houghton disliked.

Assigned to direct Houghton's story was Timothy Combe; he had worked on The Silurians the previous year. Combe's concerns about the availability of Asian actors for the serial were assuaged somewhat when he learned that Houghton was married to Pik-Sen Lim, whom he cast as Chin Lee. An unusual complication occurred when costume designer Bobi Bartlett outsourced the assembly of the Stangmoor Prison convict uniforms to a freelancer who had recently left the well-regarded Bermans & Nathans. The individual was then arrested for theft from his former employers, without completing his Doctor Who work. Bartlett was forced to visit him in prison in order to discover the location of the materials, which were then given to another costumier to finish the job. Barry Letts was able to secure the assistance of both the Royal Air Force and the Royal Artillery, after making mention of the Army's considerable help on The Invasion two years earlier.

Location filming mostly took place in Kent, with three days -- October 26th to 28th -- spent at Dover Castle, which doubled as Stangmoor. Combe had hoped to use a real prison, but had been denied permission. In order to justify using the eleventh-century fortification in Dover, the Doctor was given a line to explain that Stangmoor had originally been a medieval fortress. On the 28th, Combe filmed the evacuation of the Doctor and Jo via helicopter. The director had added this element to Houghton's scripts in order to add credibility to the escape, despite the substantial expense involved.

Part of October 28th was also used to film the hijacking of the Thunderbolt missile, on Archer's Court Road in Whitfield. For this scene, Combe's team was loaned a real -- but unprimed -- Thunderbird 2 SAGW missile from the Royal Artillery's 36th Heavy Air Defence Regiment out of Horseshoe Barracks in Shoeburyness, Essex. A number of troops from the Regiment were also made available to play the Master's men. John Levene injured his leg during the Whitfield filming, after falling badly while being hauled out of the truck's cab by a stuntman playing one of the criminals.

October 29th saw cast and crew travel to RAF Manston, for material set at Stanham Airfield. Now it was the turn of Royal Air Force personnel to play the Master's men but, unfortunately, they were dressed in military raiment rather than convict garb. This prompted Dicks to amend Episode Five, adding the Master's reference to mercenaries wearing fake uniforms. Roadway scenes were then recorded on October 30th, at both RAF Swingate in Dover and Pineham Road in Pineham. This should have completed all the Kentish material but, by this time, it was known that Combe had not captured enough shots of the Episode Five battle at Stangmoor, with Letts particularly concerned by the lack of close-ups. As a result, an additional day's filming took place on Hallowe'en at Dover Castle, with members of the production team -- including Combe himself -- stepping in front of the camera.

Filming resumed in Kensington, London, initially at Cornwall Gardens on November 2nd and 3rd for the exterior of the United Nations Intelligence Taskforce (UNIT) headquarters. Combe's offspring and Bobi Bartlett's son were the children on the playground in the scene where Chin Lee burned the purloined papers. Later on the 3rd, Benton's pursuit of Chin Lee took place along nearby Cornwall Gardens Walk; John Levene's involvement came about when another actor fell ill, and so the scene was rewritten to feature Benton. Recording on the 3rd concluded at the Commonwealth Institute, which served as the site of the peace conference; this work continued on November 4th. Unfortunately, some of this material -- involving the Doctor speaking with Fu Peng -- had to be discarded when Combe became dissatisfied with actor Andy Ho. Kristopher Kum, Ho's agent, quickly agreed to fill in as Fu Peng when production shifted to the studio.

For the scene in which the Doctor spoke Hokkien Chinese, Jon Pertwee was coached by Pik-Sen Lim

As usual, The Mind Of Evil was recorded fortnightly on Fridays and Saturdays at BBC Television Centre in White City, London with Combe following the same pattern Letts had adopted when directing the preceding serial, Terror Of The Autons. As such, the Fridays were dedicated to effects-heavy scenes, with the remainder of the corresponding installments taped on the Saturdays. The first session took place on November 21st and 22nd in Studio 3, and saw Episodes One and Two taped alongside the material in the Chinese delegation's suite for Episode Three. For the scene in which the Doctor spoke Hokkien Chinese, Pertwee was coached by Pik-Sen Lim, although he still encountered sufficient difficulty that the lines had to be simplified.

The second studio session, on December 4th and 5th, saw production move to TC6, where Episodes Three and Four were scheduled to be completed. During a fight scene taped on the Friday, Katy Manning injured her back when an extra fell against her. A rare glimpse of the past was provided via BBC photographs used to represent the illusory monsters menacing the Doctor. These included a Dalek (from The Dalek Invasion Of Earth), Koquillion (from The Rescue), the Ice Warrior leader Slaar (from The Seeds Of Death), a Zarbi (from The Web Planet), a War Machine (from The War Machines), a Silurian (from The Silurians), an Ice Warrior (also from The Seeds Of Death) and a Cyberman (from The Invasion). Considered but unused were the Slyther (from The Dalek Invasion Of Earth), the Servo Robot (from The Wheel In Space) and a Sensorite (from The Sensorites).

Unfortunately, Combe ran out of time during this session and was unable to record the Episode Three scenes of the Master and Mailer instigating the prison riot. They were postponed to the final recording block, on December 18th and 19th, for which the production reverted to TC3. A more serious mishap occurred when there was a structural collapse at the film processing lab handling The Mind Of Evil. Some of the footage recorded on October 30th was irreparably damaged, impacting Combe's choice of shots for the convoy element of the narrative.

Regrettably, a number of extra expenses -- most notably the additional day at Dover Castle and the last-minute addition of the Doctor and Jo's escape via helicopter -- pushed The Mind Of Evil well over-budget, in spite of the fact that considerable funds had already been allocated. Consequently, Letts elected not to use Timothy Combe again on future Doctor Who serials. The Mind Of Evil was also Houghton's final contribution to the programme.

Throughout the transmission of Terror Of The Autons, Doctor Who had been followed by a news update and then Here's Lucy but, during The Mind Of Evil, only Episode Two led into the Lucille Ball sitcom. Episode One, aired on January 30th, was instead followed by Lucky 13, an interview with Apollo 13 commander Jim Lovell timed to coincide with the launch of Apollo 14 the next day. Then, with the transmission of Episode Three on February 13th, Here's Lucy was replaced with a new American comedy: the soon-to-be classic The Mary Tyler Moore Show.

• Doctor Who Magazine #208, 19th January 1994, “Archive: The Mind Of Evil” by Andrew Pixley, Marvel Comics UK Ltd.
• Doctor Who Magazine #563, May 2021, “Reunited With UNIT” by Rossa McPhillips, Panini UK Ltd.
• Doctor Who Magazine Special Edition #2, 5th September 2002, “Something Old, Something New” by Andrew Pixley, Panini Publishing Ltd.
• Doctor Who: The Complete History #16, 2018, “Story 56: The Mind Of Evil”, edited by John Ainsworth, Hachette Partworks Ltd.
• Doctor Who: The Handbook: The Third Doctor by David J Howe and Stephen James Walker (1996), Virgin Publishing.
• Doctor Who: The Seventies by David J Howe, Mark Stammers and Stephen James Walker (1994), Virgin Publishing.