The Doctor has a premonition that he will assassinate the President of the High Council of the Time Lords. Returning to Gallifrey, he finds himself unable to prevent the murder, nor his own arrest. Only by declaring his candidacy for the presidency does the Doctor buy the time he needs to investigate, with the reluctant assistance of Castellan Spandrell. The trail of clues leads to the Master: now disfigured, dying and vengeful, he has somehow tapped into the extraordinarily advanced computer network which guides the Time Lords. To stop his old enemy, the Doctor must risk his life in the surreal landscape of the Matrix.

By early 1976 it was known that Elisabeth Sladen, who played Sarah Jane Smith, would be leaving Doctor Who in Season Fourteen's second serial. Producer Philip Hinchcliffe and script editor Robert Holmes turned their thoughts to the Doctor's next companion, but neither had a clear vision of what this character should be like. They also had to contend with Tom Baker's desire to portray a Doctor travelling alone, talking to himself to advance the narrative and explain the plot. Eventually, Hinchcliffe and Holmes agreed that a permanent companion would not be introduced immediately after Sarah's departure in The Hand Of Fear. They felt that, for a time, writers could invent one-off characters to assist the Doctor in each adventure. Holmes also thought that this approach would help convince Baker of the value of an ongoing companion.

It was determined that the first of these companionless serials should be written by Holmes himself. Although the BBC typically discouraged script editors from writing for their own programme, the unusual requirement warranted an exception, and formal clearance was eventually sought on May 27th. Hinchcliffe suggested a storyline in the vein of conspiracy thrillers such as the paranoiac 1962 Frank Sinatra film The Manchurian Candidate. Instead of an American soldier attempting to assassinate the President after being brainwashed by the Chinese, for Doctor Who it was thought that the Doctor might be framed for murder as part of a sinister cover-up.

Robert Holmes disliked the godlike image projected by the Time Lords in previous stories

Holmes decided that such an adventure could best take place within the context of Time Lord society which, to that point, had been subjected to only superficial exploration. He disliked the godlike image projected by the Time Lords in previous stories and concluded that this should be revealed as a mask, beneath which lay a society which was actually decaying and corrupt. Holmes felt that the seeds of this approach had already been laid by the introduction of so many renegade Time Lords -- not just the Doctor, but also the Meddling Monk (in 1965's The Time Meddler and The Daleks' Master Plan), the War Chief (1969's The War Games), Omega (1972's The Three Doctors), Morbius (1976's The Brain Of Morbius) and, most infamously, the Master, whom Holmes had introduced in 1971's Terror Of The Autons.

Indeed, the Gallifreyan setting suggested that the Master would be a suitable villain for Holmes' story, following an absence from Doctor Who since Frontier In Space in 1973. Although an adventure had been planned for 1974 in which the Master would have died in an act of apparent self-sacrifice, actor Roger Delgado was killed in a car crash before it could be made. But while Hinchcliffe and Holmes felt that sufficient time had now passed to resurrect the character, they were also contemplating leaving Doctor Who at the end of Season Fourteen. Both men felt that they had done as much with the show as they could, and were developing a more adult science-fiction programme called Lituvin 40 as a potential next project. As such, they decided to reintroduce the Master in a transitional state, so that their successors would not be saddled with a version of the character they might find unsuitable. Instead of the suave, silky Time Lord played by Delgado, the new Master would therefore be a desperate, skeletal creature barely clinging to life.

Holmes was working on his scripts -- initially called “The Dangerous Assassin” and then The Deadly Assassin -- by April 1976. Since the story was partly inspired by the conspiracy theories which had abounded after the deaths of American personalities such as President John F Kennedy, Holmes incorporated several digs at the United States. The Time Lord organisation which had occasionally been seen to manipulate the Doctor in recent years was christened the Celestial Intervention Agency, sharing its initials with the Central Intelligence Agency, which was often accused of being at the centre of various conspiracies. The Doctor's line about “vaporisation without representation” spoofed a similar American slogan about taxation, which had been popularised during the War of Independence. Meanwhile, Holmes' reimagining of Time Lord society established several hallmarks of Doctor Who mythology, including Rassilon, the Eye of Harmony, the Matrix, the Prydonian chapter, artron energy, and the notion that Time Lords could regenerate only twelve times.

Robert Holmes came up with the idea of the Doctor being accompanied by a Dickensian street urchin

For a while, it was thought that Episode Four might introduce a new companion character after all. Holmes had come up with the idea of the Doctor being accompanied by a Dickensian street urchin, whom he would mentor in the manner of Henry Higgins and Eliza Doolittle. Originating in George Bernard Shaw's 1913 play Pygmalion and popularised by its 1964 film adaptation My Fair Lady, Higgins was a brilliant but aloof professor, and Doolittle was the clever but unsophisticated waif he took under his wing. As such, Holmes considered setting the climactic encounter between the Doctor and the Master in Victorian London. However, the new companion's introduction was eventually postponed until later in the season.

The director assigned to The Deadly Assassin was David Maloney, whose last Doctor Who work had been on Planet Of Evil a year earlier. Maloney worked closely with designer Roger Murray-Leach and costume designer James Acheson to give Gallifrey a consistent, cohesive look. Although he would eventually be replaced on the serial by Joan Ellacott, Acheson contributed the Time Lords' distinctive high-collared silhouette, while Murray-Leach reused a symbol he had designed for 1975's Revenge Of The Cybermen as the Prydonian seal. Both would become enduring elements of Gallifrey's portrayal in Doctor Who, with the latter subsequently coming to be known as the “Seal of Rassilon”.

Eager to take Doctor Who into unexplored territory, Hinchcliffe requested that Holmes write part of The Deadly Assassin as a surrealist nightmare, to be captured entirely on film. This inspired the extended duel between the Doctor and Chancellor Goth within the confines of the Matrix, which formed the lion's share of Episode Three. It also consumed the entirety of the serial's location filming, which began at Betchworth Quarry in Betchworth, Surrey. Material on the plains and in the wasteland was completed there from July 26th to 28th.

Later on the 28th, cast and crew moved to the Royal Alexandra and Albert School in Merstham, Surrey. Sequences in the jungle environment were recorded there through July 30th. Because the pond water was too murky, the shot of Goth trying to drown the Doctor was actually performed in the school's swimming pool. Work on the 30th concluded with the biplane attack, captured at Wycombe Air Park in High Wycombe, Buckinghamshire. This locale was a late substitution for the planned venue, the Redhill Aerodrome in Redhill, Surrey. The sequence itself was an homage to the classic scene in the 1959 Alfred Hitchcock thriller North By Northwest, in which bullets fired from a crop duster rained down on Cary Grant's character.

Most of the actors in The Deadly Assassin appeared only during the studio recordings. This included Peter Pratt, whom Maloney cast as the decayed Master. A skilled voice artiste and opera performer who had been a friend of Delgado's, Pratt's other television credits included Z Cars and Play For Today. Pratt's uncomfortable mask was originally fitted with tubes which were intended to give the impression of fluid circulating around the Master's atrophied skull. However, this element was discarded when it was discovered that the effect was not visible under the studio lights.

The first studio session for The Deadly Assassin took place from August 15th to 17th at BBC Television Centre Studio 3 in White City, London. Maloney principally dealt with Episodes One and Two, beginning with scenes in the TARDIS, the museum and the cloisters on the 15th. The next day took in material in the chancellery, the lift and the Panopticon, including the climactic fight in Episode Four. On the 17th, the remaining Panopticon sequences were completed, alongside those in the service gallery and the records room.

The second studio block was originally scheduled for August 30th and 31st, but was ultimately shifted to September 1st and 2nd. It was held in TC8, and was largely devoted to Episodes Three and Four. The first day saw the recording of the remaining records room material, as well as sequences set in the detention cell (from Episode Two) and the Master's sanctuary, some of which hailed from the first two installments. Taping on the final day included material in the chancellery, the vault, the chimney and the museum; the latter included a remount of the arrival of the TARDIS for Episode One.

As scripted, the cliffhanger to Episode Two was the Doctor's fall after being attacked by the samurai. When the installment was found to be running very short, it was extended up to the train sequence. Late changes to Episode Three saw the deletion of a spider crawling along the Doctor's arm -- which Bill Slater, the Head of Serials, deemed to be too disturbing for younger viewers -- and the introduction of a freeze frame at the cliffhanger, with Goth holding the Doctor's head below the water. At one point, it was planned to include a humorous title card at the end of Episode Four reading, “We thank the High Court of Time Lords and the Keeper of the Records for their help and co-operation.” Ultimately, however, it was removed for fear that it lampooned the production too much.

After The Deadly Assassin, Doctor Who went on hiatus for 5 weeks over the Christmas period

After some minor variation during the early part of Season Fourteen, Doctor Who's timeslot stabilised at 6.05pm throughout The Deadly Assassin. The programme continued to be bookended by The Basil Brush Show and Bruce Forsyth And The Generation Game. With the adventure's conclusion on November 20th, Doctor Who went on hiatus for five weeks over the Christmas period -- up to that point, the longest gap in transmission to be incorporated within a season.

But even though no new episodes of Doctor Who aired during December, the programme -- and The Deadly Assassin -- remained a hot topic of conversation. Mary Whitehouse of the National Viewers' and Listeners' Association had been a thorn in the show's side throughout the Seventies, and had escalated her attacks on the suitability of its content since the start of 1975. Whitehouse was at her most strident in her condemnation of the protracted drowning sequence at the end of Episode Three; she even quoted one child who had allegedly told his mother that he would do the same to his younger brother the next time the boy angered him.

Whereas Whitehouse's litany of complaints had been largely dismissed in the past, on this occasion she was successful in cajoling an apology from Sir Charles Curran, Director General of the BBC. The master tape of Episode Three itself was edited to expunge the freeze-frame sequence altogether, meaning that the BBC no longer retained a complete copy of the original broadcast; fortunately, it was subsequently reconstructed using off-air home video recordings. It appears that Whitehouse was mollified by Curran's response, together with Doctor Who's shift to a later timeslot when it returned in January 1977 and the change in tone which accompanied the arrival of Graham Williams as producer for Season Fifteen. She thereafter turned her attentions elsewhere, and it would be nearly a decade before similar complaints were levied at Doctor Who.

• Doctor Who Magazine #187, 10th June 1992, “Archive: The Deadly Assassin” by Andrew Pixley, Marvel Comics UK Ltd.
• Doctor Who Magazine Special Edition #8, 1st September 2004, “Take It To The Limit” by Andrew Pixley, Panini Publishing Ltd.
• Doctor Who: The Complete History #26, 2015, “Story 88: The Deadly Assassin”, edited by John Ainsworth, 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 #18, August 1989, “Production” edited by Justin Richards and Peter Anghelides, Cybermark Services.