The Master, posing as Professor Keller, has created a device he purports will remove the negative impulses from the brains of convicted criminals. The Keller Machine in fact contains an alien mind parasite which turns the convicts into servants of the Master. With their help, the evil Time Lord hijacks a nerve missile, with which he intends to hold a world peace conference hostage.

Don Houghton had impressed the Doctor Who production office with his efficiency in writing Inferno for Season Seven. As a result, on June 29th, 1970 he was commissioned to write a treatment for a second serial called “The Pandora Machine” (some sources suggest the working titles “Man Hours” and “The Pandora Box” or “The Pandora's Box” may also have been used). Scripts for the story were requested soon thereafter, on August 6th. As with all Season Eight adventures, Houghton was asked to incorporate the evil Time Lord called the Master, created by producer Barry Letts and script editor Terrance Dicks, into his storyline.

Houghton was initially inspired by the film A Clockwork Orange to write a story in which a machine alters criminal minds with the ostensible aim of rehabilitating them. He was concerned that this would not be substantial enough to flesh out six episodes, however, and adopted the suggestion of his wife, Asian actress Pik-Sen Lim, of including a subplot involving intrigue at an international peace conference.

“The Pandora Machine” underwent several alterations during late summer and early autumn. The Master's pseudonym was originally Emil Dalbiac before the surname was changed to Keller (the first name was also sometimes spelt “Emile”), while the Keller Machine itself was conceived as the Malusyphus box. Corporal Bell's last name was initially Bates, and the prison governor was called Major Victor Camford in the scripts. Another character, Lenny Vosper, was named after Houghton's agents, Margery Vosper Ltd.

Barnham was killed much earlier, and the Brigadier was captured at Stangmoor Prison along with the Doctor and Jo; under the influence of the Master's hypnotism, he had the missile's route changed to enable the Master's men to hijack it. One of the weapons used by the Keller Machine was the image of a Gorgon-like monster, which the Doctor eventually destroys by showing it its own reflection in the mirror of the missile transport. As well, Houghton originally played up the tension between the United States and China, with Chin Lee attempting to frame an American delegate for Chang Teik's murder. Part two's cliffhanger involved Chin Lee trying to blow up the peace conference using an explosive called kredalite.

In early September, Serial FFF gained its new title of The Mind Of Evil, of which Houghton was not particularly enamoured. The director assigned to The Mind Of Evil was Timothy Combe, who had worked on The Silurians a year earlier. Combe's fears about the availability of Asian actors for the story were assuaged somewhat when he learned that Houghton was married to Pik-Sen Lim, whom he cast as Chin Lee.

Location filming mostly took place in Kent, with three days -- October 26th to 28th -- spent at Dover Castle, which doubled as Stangmoor (the government had refused Combe permission to visit a real prison). Part of the 28th was also used to film the hijacking of the Thunderbolt missile, on Archers Court Road in Whitfield. For this scene, the production office had been able to secure the loan of a real (but unprimed) Thunderbird 2 SAGW missile from the 36th Heavy Air Defence Regiment out of Horseshoe Barracks in Shoeburyness, Essex, in addition to a number of troops who would play the Master's men (eight marines had also been made available for the Dover Castle sequences). Unfortunately, however, the soldiers were dressed in military raiment rather than convict garb, prompting Dicks to add a line to episode five wherein Yates explained that the criminals had been wearing fake uniforms. John Levene injured his leg during the filming of these sequences 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 Alland Grange, RAF Manston, for the scenes set at Stanham Airfield. The Doctor and Jo's evacuation by helicopter, filmed on this day, was a late addition to the scripts. Roadway material was then performed 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 (most notably close-ups) of the episode five battle at Stangmoor Prison. As a result, an additional day's filming took place at Dover Castle on Hallowe'en with members of the production team -- including Combe himself -- stepping in front of the camera.

Location work then resumed back in London on November 2nd and 3rd, where Cornwall Gardens in Kensington served as UNIT headquarters. Filming concluded with material set at the Commonwealth Institute in Kensington, serving as the site of the peace conference, on the 3rd and 4th. Unfortunately, some of this material -- involving the Doctor speaking with Fu Peng -- had to be discarded when Andy Ho, the actor originally hired to play Fu Peng, was hastily replaced by Kristopher Kum during rehearsals a week later.

All three two-day studio sessions took place on Fridays and Saturdays. For each session, Combe elected to spend the Friday recording inserts and then tape the main action on the Saturday. The first session took place on November 21st and 22nd in BBC Television Centre Studio 3, and saw the completion of episodes one and two, as well as some inserts for part three. For the scene in which the Doctor speaks Chinese (specifically, the Hokkian dialect), Pertwee was coached by Pik-Sen Lim, although the actor still experienced enough difficulty that the lines had to be simplified somewhat.

The second studio session, on December 4th and 5th, saw production move to TC6, where the remainder of episodes three and four was scheduled to be recorded. During a fight scene taped on the 4th, Katy Manning injured her back when an extra fell against her. A number of BBC photographs were used to represent the illusory monsters which menace the Doctor at the end of part three. These included a Dalek (from The Dalek Invasion Of Earth), Koquillion (from The Rescue), the Ice Lord 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). Also 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 complete the episode three scenes of the Master and Mailer instigating the prison riot. These were postponed until the final recording days, December 11th and 12th, for which the production reverted to TC3.

Regrettably, a number of extra expenses -- most notably the additional day at Dover Castle and the last-minute addition of the helicopter to the climax of episode six -- pushed the already costly The Mind Of Evil well over-budget. Consequently Letts elected not to use Timothy Combe again on future Doctor Who serials. Combe went on to work on programmes such as Z Cars and The Brothers before joining BBC Videograms as a drama producer. One of the projects he worked on during this time was a video recounting the history of Doctor Who (with Dicks' assistance), but nothing came of it. Combe left the BBC in 1981 and subsequently worked as an agent.

The Mind Of Evil was also Houghton's final contribution to Doctor Who. Soon thereafter, he joined Hammer Films as a writer and producer, contributing scripts to a number of movies such as The Satanic Rites Of Dracula. For television, he wrote episodes of programmes including Sapphire And Steel, and created the Scottish soap opera Take The High Road. In the late Seventies, Houghton also published two novels, entitled Blood Brigade and Column Of Thieves. He died following an illness in July 1991.

• Doctor Who: The Handbook: The Third Doctor by David J Howe and Stephen James Walker (1996), Virgin Publishing, ISBN 0 426 20486 7.
• 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 #208, 19th January 1994, “Archive: The Mind Of Evil” by Andrew Pixley, Marvel Comics UK Ltd.
• Doctor Who Magazine Special Edition #2, 5th September 2002, “Something Old, Something New” by Andrew Pixley, Panini Publishing Ltd.