Doctor Who
Season 16
Cover of the 2008 Region 2 DVD release
Country of originUnited Kingdom
No. of stories6
No. of episodes26
Original networkBBC1
Original release2 September 1978 (1978-09-02) –
24 February 1979 (1979-02-24)
Season chronology
← Previous
Season 15
Next →
Season 17
List of episodes

The sixteenth season of British science fiction television series Doctor Who, known collectively as The Key to Time, began on 2 September 1978 with The Ribos Operation, and ended with The Armageddon Factor. The arc was originally conceived by producer Graham Williams, who had proposed it as part of his application for the producer's job in 1976.[1] The name refers to the powerful artefact, the segments of which are what the Fourth Doctor and his companions, Romana and K9, search for during the season. Anthony Read was the script editor until the final story, when Douglas Adams became the new script editor.


A figure calling himself the White Guardian commissions the Doctor and K9, assisted by a new companion, the Time Lady Romana, to find the six segments of the Key to Time, a cosmic artefact resembling a perfect cube that maintains the equilibrium of the universe. Since it is too powerful for any single being to possess, it has been split into six different segments and scattered across space and time, disguised by the raw elemental power within them into any shape or size. However, since the forces balancing the universe are so upset, the White Guardian needs to recover the segments of the Key to stop the universe so that he can restore the balance. The White Guardian also warns the Doctor of the Black Guardian, who also wishes to obtain the Key to Time for his own purposes. In the final episode, the Black Guardian, disguised as the White Guardian, attempts to take the Key from the Doctor. However, the Doctor sees through the figure's charade and orders the segments of the Key to Time to once again become scattered across all of time and space, bar the sixth, which he reinstates as Princess Astra. Afterward, the Doctor decides to install a device called a randomiser into the TARDIS' navigation system for a period of time to make his following voyages unpredictable to evade the Black Guardian.

The six segments

  1. The first segment is disguised as a lump of Jethrik on the planet Ribos.
  2. The second is the planet Calufrax, shrunk to miniature size by the space-hopping pirate planet Zanak.
  3. The third is the Great Seal of Diplos, which has been stolen by a criminal of that planet.
  4. The fourth is part of a statue on the planet Tara.
  5. The fifth has been consumed by the squid Kroll, causing it to turn into a gigantic monster.
  6. The final segment is a female humanoid – Princess Astra.


See also: List of Doctor Who cast members

Main cast

Guest cast

Tom Baker continued his role as The Fourth Doctor, and saw the introduction of Romana played by Mary Tamm. This season was the only one to feature Tamm as the first incarnation of Romana, as Tamm left the programme after only one season because she felt that the character had reverted to the traditional assistant role and could not be developed further.[2] The second incarnation, played by Lalla Ward (who also appeared in this season as Princess Astra), began her run in the first serial of the next season (Destiny of the Daleks).


See also: List of Doctor Who episodes (1963–1989)

Douglas Adams took over as script editor from Anthony Read for The Armageddon Factor. Season 16 consists of one long story arc encompassing six separate, linked stories.

No. in
Serial titleEpisode titlesDirected byWritten byOriginal air dateProd.
UK viewers
(millions) [3]
981The Ribos Operation"Part One"George Spenton-FosterRobert Holmes2 September 1978 (1978-09-02)5A8.359
"Part Two"9 September 1978 (1978-09-09)8.1
"Part Three"16 September 1978 (1978-09-16)7.9
"Part Four"23 September 1978 (1978-09-23)8.267
The Doctor is recruited by the White Guardian to seek the six segments of the Key to Time. The quest for the first segment takes them to Ribos, a medieval planet that galactic confidence trickster Garron is trying to sell to the Graff Vynda-K.
992The Pirate Planet"Part One"Pennant RobertsDouglas Adams30 September 1978 (1978-09-30)5B9.161
"Part Two"7 October 1978 (1978-10-07)7.4
"Part Three"14 October 1978 (1978-10-14)8.264
"Part Four"21 October 1978 (1978-10-21)8.464
The quest for the second segment takes them to the planet Zanak, which has been hollowed out and fitted with hyperspace engines, allowing its insane half-robot Captain to materialise it around other smaller planets and plunder their resources.
1003The Stones of Blood"Part One"Darrol BlakeDavid Fisher28 October 1978 (1978-10-28)5C8.6
"Part Two"4 November 1978 (1978-11-04)6.6
"Part Three"11 November 1978 (1978-11-11)9.3
"Part Four"18 November 1978 (1978-11-18)7.667
The quest for the third segment takes them to Earth, where an ancient criminal who stole the segment disguises herself as the Celtic goddess Cailleach.
1014The Androids of Tara"Part One"Michael HayesDavid Fisher25 November 1978 (1978-11-25)5D9.5
"Part Two"2 December 1978 (1978-12-02)10.165
"Part Three"9 December 1978 (1978-12-09)8.9
"Part Four"16 December 1978 (1978-12-16)9.066
The quest for the fourth segment takes them to the planet Tara. The Fourth Doctor and Romana find themselves embroiled in the political games of the planet Tara, where doubles, android or otherwise, complicate the coronation of Prince Reynart.
1025The Power of Kroll"Part One"Norman StewartRobert Holmes23 December 1978 (1978-12-23)5E6.5
"Part Two"30 December 1978 (1978-12-30)12.4
"Part Three"6 January 1979 (1979-01-06)8.9
"Part Four"13 January 1979 (1979-01-13)9.963
The quest for the fifth segment takes them to the third moon of Delta Magna, caught in the middle of a dispute between the crew of a methane refinery and the natives (known as 'Swampies').
1036The Armageddon Factor"Part One"Michael HayesBob Baker and Dave Martin20 January 1979 (1979-01-20)5F7.565
"Part Two"27 January 1979 (1979-01-27)8.8
"Part Three"3 February 1979 (1979-02-03)7.8
"Part Four"10 February 1979 (1979-02-10)8.6
"Part Five"17 February 1979 (1979-02-17)8.6
"Part Six"24 February 1979 (1979-02-24)9.666
The quest for the sixth and final segment takes them to Atrios, a world at war with its planetary neighbour Zeos. But the Black Guardian is closing in.


The Key to Time was broadcast from 2 September 1978 to 24 February 1979.

Home media

See also: List of Doctor Who home video releases and List of other Doctor Who home video releases

VHS releases

Season Story no. Serial name Number and duration
of episodes
UK release date Australia release date USA/Canada release date
16 98 The Ribos Operation 4 x 25 min April 1995 June 1995 September 1996
99 The Pirate Planet 4 x 25 min April 1995 June 1995 September 1996
100 The Stones of Blood 4 x 25 min May 1995 July 1995 March 1996
101 The Androids of Tara 4 x 25 min May 1995 July 1995 March 1996
102 The Power of Kroll 4 x 25 min June 1995 August 1995 September 1996
103 The Armageddon Factor 6 x 25 min June 1995 August 1995 September 1996

DVD and Blu-ray releases

All releases are for DVD

Season Story no. Serial name Number and duration
of episodes
R2 release date R4 release date R1 release date
16 98–103 The Key to Time[a] 26 × 25 min. 1 October 2002[4]
The Key to Time:

The Ribos Operation
The Pirate Planet
The Stones of Blood
The Androids of Tara
The Power of Kroll
The Armageddon Factor

26 × 25 min.[b] 24 September 2007[c]
16 November 2009[d][5]
7 November 2007[6] 3 March 2009[7]
  1. ^ Stories available in Region 1 only, with minimal restoration and extras (individually or as a box set)
  2. ^ Special edition. Known collectively as The Key to Time. Only available as a box set in Regions 2 and 4. Stories available individually or as a box set in Region 1. All serials contain four episodes each, apart from The Armageddon Factor, which has six episodes.
  3. ^ Limited release
  4. ^ Re-release

In print

See also: List of Doctor Who novelisations

Season Story no. Library no.[a] Novelisation title Author Hardcover
release date[b]
release date[c]
release date[d]
16 098 52 Doctor Who and the Ribos Operation Ian Marter 13 December 1979 3 March 2011
099 The Pirate Planet (unabridged) James Goss 5 January 2017 1 February 2018 [e] 5 January 2017
Doctor Who and the Pirate Planet (abridged) 11 March 2021
100 59 Doctor Who and the Stones of Blood Terrance Dicks 20 March 1980
The Stones of Blood David Fisher 14 July 2022[8] 5 May 2011
101 3 Doctor Who and the Androids of Tara Terrance Dicks 24 April 1980
The Androids of Tara David Fisher 14 July 2022[9] 5 July 2012
102 49 Doctor Who and the Power of Kroll Terrance Dicks 29 May 1980 7 October 2021
103 5 Doctor Who and the Armageddon Factor 26 June 1980 6 June 2019
  1. ^ Number in Target's Doctor Who Library, if applicable
  2. ^ Published by Target's parent companies (Allen Wingate, W. H. Allen, BBC Books) unless otherwise indicated
  3. ^ Published by Target Books (or by BBC Books under the Target Collection umbrella) unless otherwise indicated
  4. ^ Unabridged from BBC Audio/AudioGo unless otherwise indicated
  5. ^ BBC Books


  1. ^ "Season 16". Doctor Who Classic Episode Guide. BBC. Retrieved 10 August 2007.
  2. ^ "". Archived from the original on 25 July 2008.
  3. ^ a b "Ratings Guide". Doctor Who News. Retrieved 27 December 2014.
  4. ^ "Doctor Who - The Key to Time Collection". 1 October 2002. Retrieved 16 November 2017 – via Amazon.
  5. ^ Smith 2014, The Key to Time.
  6. ^ "Doctor Who The Key To Time Box Set by BBC - Shop Online for Movies, DVDs in Australia". Fishpond. Archived from the original on 17 November 2017. Retrieved 16 November 2017.
  7. ^ "Doctor Who: The Key to Time". 3 March 2009. Retrieved 16 November 2017 – via Amazon.
  8. ^ ASIN 178594794X.
  9. ^ ASIN 1785947923.