The twenty-third season of British science fiction television series Doctor Who, known collectively as The Trial of a Time Lord, aired in weekly episodes from 6 September to 6 December 1986. It contained four adventures: The Mysterious Planet, Mindwarp, Terror of the Vervoids and The Ultimate Foe; the season also marked the final regular appearance of Colin Baker as the Sixth Doctor.

The idea for the serial stemmed from several production changes to Doctor Who, such as reduced screen time for the season and an instruction from BBC1 controller Michael Grade that the series needed to contain less violence and more humour.^[1] Several problems occurred during production, including the death of scriptwriter Robert Holmes and the resignation of script editor Eric Saward. When it ended, Baker was dismissed from the role on the orders of Grade.^[2][3] John Nathan-Turner produced the series.

In the serial, the Sixth Doctor is tried by the High Council of Time Lords for breaking several of the laws of Gallifrey, the Time Lords' home world, including interference with outside worlds and genocide. A mysterious character called the Valeyard acts as prosecutor. In the first two chapters (The Mysterious Planet and Mindwarp) events from the Doctor's past and present are submitted as evidence of his guilt. The third chapter (Terror of the Vervoids) presents future events in the Doctor's defence. In the concluding chapter (The Ultimate Foe) the Doctor's trial is halted, and the Doctor confronts the Valeyard and his old rival, the Master, in order to clear his name and to save the High Council.

The series remained at once-weekly Saturday broadcasts. All episodes were 25 minutes long, with the exception of Part 14, which ran for just under 30 minutes. This running time was the result of a plea by John Nathan-Turner to his superiors at the BBC that the serial's final episode needed the additional three minutes to conclude the story properly. Although there were now 14 episodes in the season, the total running time was overall reduced since the episodes were just over half as long.

Robert Holmes was commissioned to write the first and final chapters of the serial. His draft of the first chapter, The Mysterious Planet, was criticised by BBC Head of Series and Serials, Jonathan Powell, for its comedic content, contradicting the BBC controller's request for a more humorous series – which took away confidence from Holmes.

The second chapter, Mindwarp, was written by Philip Martin., whose character Sil, introduced in the previous season's Vengeance on Varos, was popular among the production team, who asked Martin to feature the character in the ultimately-cancelled story Mission to Magnus. Nathan-Turner asked Martin to include Sil in his chapter, and asked confidentially for Peri to be killed in accordance with Bryant's wishes to leave the show with a bang.^{[citation needed]}

The third chapter was originally to be interlinked with the fourth. Holmes was originally asked to write it, but declined, citing a dislike of six-part serials. After rejecting submissions by Christopher H. Bidmead and PJ Hammond,^[1] Nathan-Turner approached husband-and-wife writing team Pip and Jane Baker to write a studio-based serial, Terror of the Vervoids.^{[14][citation needed]}

Holmes was unable to finish writing the fourth chapter, originally called Time Inc., before his death from a liver illness and Hepatitis B on 24 May 1986, aged 59.^{[14][citation needed]} The Bakers were commissioned by Nathan-Turner, at short notice, to write a new version of the episode, after script editor Eric Saward withdrew his permission for his original version of Part 14 to be used: the original ending would have featured a fight to the death in a time vent, between the Doctor and the Valeyard, as part of a cliff-hanger ending to the season, which horrified Nathan-Turner, as he feared that to use such a downbeat ending would have provided an excuse for the BBC management to cancel the series altogether. Subsequently, the title was later changed to The Ultimate Foe.^{[14][citation needed]}

Filming of the serial began on 7 April 1986 and ended on 14 August that year.^{[citation needed]} For the opening sequence, Nathan-Turner commissioned a 45-second model shot that cost over £8,000, which at that time was the highest amount of money spent on a single special-effects sequence in the history of the series. The outdoor sequences in The Mysterious Planet were filmed in mid-April in Queen Elizabeth Country Park, and studio work followed on 24 April and 10 May.^{[citation needed]} Studio work for Mindwarp took place from 27 to 29 May and 11–13 June, and location shots were filmed in Brighton from 15 to 16 June.^{[citation needed]} Terror of the Vervoids and The Ultimate Foe were produced simultaneously; production began with location filming for the latter in late June,^{[citation needed]} before returning to the studio to film scenes for both chapters on 16–17 July.^{[citation needed]} Terror of the Vervoids was the last chapter to be completed, with studio work taking place from 30 July–1 August and from 12 to 14 August.^{[citation needed]}

Public reaction to The Trial of a Time Lord was mixed. Although the Audience Appreciation figures had improved since the previous season — the lowest figure was 66% for Parts Seven and Nine^[4] and the highest was 72% for Parts One, Four and Eight.^[4]— the viewing figures were lower.^[4]

The Trial of a Time Lord was broadcast from 6 September to 6 December 1986.

Novelisations of the originally planned season 23

• ^a Although the following serial Time and the Rani was the Sixth Doctor's final appearance, Baker declined an offer to return either for the entire story (as originally offered) or solely for a regeneration scene, and the part was instead portrayed by Sylvester McCoy wearing a blond wig.^{[2][citation needed]}