New to Doctor Who or returning after a break? Check out our guides designed to help you find your way!



The Power of Kroll was the fifth and penultimate serial of season 16 of Doctor Who.

It was part of the Key to Time story arc which spanned the entire season. Holmes was tasked with writing a second serial for the season (having already written The Ribos Operation, the season opener) after Ted Lewis' Shield of Zarak was not completed. Holmes was asked to create a monster of epic proportions. This worried many crew members, due to cost restrictions. The Power of Kroll was to be the fourth story of the season until it was swapped with The Androids of Tara, originally fifth.

Aware that the K9 prop would not work on swampy ground, Robert Holmes wrote out K9 from the story. However, John Leeson took the role of Dugeen after the actor originally cast, Martin Jarvis, became unavailable. (DWM 312)


The Fourth Doctor and Romana I arrive on the marsh moon of Delta Magna in search of the fifth segment of the Key to Time. They are caught in the conflict between the native Swampies and the crew of a chemical refinery. The presence of a gun runner complicates matters; to make things worse, the Swampies intend to awaken Kroll, the giant god that lives beneath the swamps.


Part one[]

In the far future, on one of the moons of Delta Magna, sits a vast methane and protein collection and manufacturing refinery. The staff of five — the bullying leader Thawn, his technical support Fenner, Harg, Dugeen and another crewman — are in charge of the operation. A green-skinned humanoid native called Mensch, one of the Delta Magnans, serves them.

These servile people once lived on the planet but were moved to the moon when the Earth Empire identified Delta Magna as commercially valuable. Sadly for the green-skinned natives, known as Swampies, this is now also the case on the moon. Thawn grows concerned about the possibility of insurrection when he learns that the environmentalist group Sons of Earth has taken an interest in the Swampies, and the gun-runner Rohm-Dutt is rumoured to be journeying to the moon. Thawn and Fenner go outside the refinery to see if he has arrived, suspecting a mysterious spacecraft which has touched down nearby is Rohm Dutt's.

The Doctor and Romana I have also arrived on the moon, searching for the penultimate segment of the Key to Time. Because of the swampy terrain, K9 is confined to the TARDIS — or "marooned", as Romana puts it. However, the Tracer they are using to locate the segment is giving some very strange readings that makes their quarry difficult to pinpoint.

They are ambushed by Swampies. Romana is taken away with Rohm Dutt and a Swampie patrol. The Doctor is found by Fenner and Thawn, shot with a stun gun and is taken to the refinery. There he displays a bewildering familiarity with methane catalysation, with the refined protein shot into space for delivery to Delta Magna. The Doctor is also intrigued by the position of Mensch, whom the refinery workers do not even regard as a properly sentient being. The Doctor slips away to try to find Romana, but she has got herself into trouble. The Swampies have tied her to a rock and are about to sacrifice her to Kroll, a giant squid creature that lives under the swamps, which they worship as a god.

Part two[]

Kroll appears from the waters. It is a vast squid – or rather a Swampie named Skart dressed as one. The Doctor unmasks the fake monster and rescues Romana.

Back at the refinery, Dugeen has detected a huge creature some two miles wide inside the swamp. The Doctor and Romana have found reference to the real Kroll in a Swampie testament. It says the enormous creature ate a previous Swampie High Priest and has made three appearances. A fourth is due soon.

Rohm-Dutt has supplied the Swampies with weapons but, when they are used against Thawn, who is searching the swamps for the Doctor, they are defective and explode whenever fired, killing one of the Swampies. Seconds later, a huge tentacle erupts from the water and seizes Mensch, dragging him to his death. In the confusion, Rohm-Dutt calls out to Thawn; his cries go unnoticed. Everyone's eyes are drawn to the vast, squid-like creature that rises from the swamp, towering above the horizon. Kroll is very real, very large and very hungry. Ranquin tries to mollify it with prayer, and Kroll disappears beneath the swamp's surface. Despite Ranquin's belief that Kroll has spared his believers, the monster has simply failed to notice the Swampies, and it is soon drawn to the refinery. One of its tentacles enters a pipeline and soon smashes its way inside, dragging crewmember Harg to his death.

Part three[]

Kroll part3


The gun-runner is taken as a prisoner to the Swampie camp. The Doctor and Romana arrive and introduce themselves to the tribe and its leader, Ranquin. The elder decrees all three "dry-foots" will die by the seventh ritual of Kroll: a painful and cumbersome affair involving stretching the victims on creepers and wood that will eventually snap their backs. In fear, Rohm-Dutt admits Thawn paid him to sell the faulty weapons to the Swampies to discredit the environmentalist Sons of Earth movement. All three are secured, and their punishment begins as a violent storm starts. The Doctor rescues himself, Romana and Rohm-Dutt with "vocal vibrations" which shatter a window above, allowing the rain to pour on the vines. This relaxes them enough for the trio to escape, pursued by the Ranquin and his tribe.

The three remaining refinery crew members decide to redirect the upcoming orbit shot against Kroll. They note the creature is on the move towards the Swampie settlement. When it reaches there, its first victim is Rohm-Dutt. The Doctor deduces that the creature hunts by vibration and uses this knowledge to keep Romana and himself safe from the tentacles. The enormous Kroll rises to the swamp's surface, dragging a Swampie to his death...


The Swampies, armed with guns, wait for Kroll to rise

Part four[]

Kroll destroys the Swampie settlement, killing many Swampies before submerging once again.

The Doctor and Romana sneak back into the refinery and are horrified; Thawn calculates the missile strike will destroy the creature and the Swampies. Both Dugeen and Fenner are appalled by Thawn's plan. When Dugeen tries to stop the missiles being fired, Thawn kills him, much to Fenner's disgust. The Doctor, however, disconnects the firing mechanism in the rocket silo. Thawn investigates and finds the two culprits, more certain than ever that they are Sons of Earth spies too. The tables are turned when the Swampies attack the breached refinery and kill Thawn. Ranquin, who has led the assault, believes the deaths of the "dry foots" will help appease Kroll, but the creature is now above the swamp again and hurling itself against the bulk of the refinery.

While Ranquin fruitlessly prays to Kroll to stop the attack, the Doctor orders all the refinery's equipment activated to try to confuse Kroll. It has the desired effect, and the attack stops. Ranquin refuses to let his faith in Kroll be shaken, however. Finding one of Kroll's tentacles, he begs his god to destroy the "dry foots". Instead, Kroll grabs Ranquin and drags him away to his doom. The remaining Swampies realise the creature is beyond appeasement.

It is not, however, beyond science. The Doctor has worked out that Kroll has been magnified to giant proportions by a segment of the Key to Time, disguised as a Swampie relic which the squid creature swallowed with an earlier High Priest. He uses the Tracer to eliminate the giant Kroll and retrieve the fifth segment of the Key.

The Doctor finally saves the refinery from destruction by aborting the automatically scheduled orbit shot, which would have caused an explosion. He tells Fenner, the only survivor of the refinery crew, to use his time wisely until a rescue mission arrives from Delta Magna and to try to understand the Swampie culture. The Doctor and Romana head back to the TARDIS with another segment of the Key in their possession.




Astronomical objects[]

The Doctor[]

  • The Doctor implies he learnt his high-pitched singing technique from Dame Nellie Melba.
  • The Doctor has seen hundreds of methane catalysing refineries.

Human colonies[]

  • The Doctor says that on Binaca-Ananda you would see a methane catalysing refinery in every town after the technicians at the refinery claim their plant is the first one ever built.

Literature from the real world[]

Psychic powers[]


  • The Doctor suggests that one of the rituals would be to hang upside down over a pit of vipers.

Songs from the Real World[]

Story notes[]

  • Working titles for this story included Moon of Death, Horror of the Swamp and The Shield of Time. It should be noted that this final working title was part of a short-lived idea that all stories in Season 16 were going to be entitled The (Something) of Time, with the last story (which eventually became The Armageddon Factor) named simply The Key to Time.
  • This was the last story written by Robert Holmes for five years until The Caves of Androzani. He regarded it as his least favourite script due to the restrictions placed upon him by Graham Williams (it had to part of a story arc, it had to feature the biggest monster in the show and he had to tone down his trademark humour).
  • This story features a guest appearance by Philip Madoc. He had been invited to play Thawn, but the invitation was withdrawn because Neil McCarthy had already accepted the role. He agreed to play Fenner when Alan Browning, who was originally cast in the role, was taken ill before the start of production and had to withdraw. Madoc was disappointed at being cast in such a minor role and never appeared on the show again.
  • John Leeson, best known as the voice of K9, appears in the story as Dugeen, partly because the location of the story rendered the use of K9 unfeasible. This marks Leeson's only on-screen appearance in Doctor Who. Coincidentally, the end of the previous story, The Androids of Tara — which sees K9 stranded in a rowing boat in the middle of Castle Gracht's moat — offered a practical demonstration of why water and K9 did not mix.
  • The BBC's Head of Serials, Graeme MacDonald, was so unimpressed by the set designs in this story that he ordered that designer Don Giles was never to work on the series again. By coincidence, part three includes a line of dialogue in which the Doctor criticises the decor of the Swampies' execution chamber and recommends its architect be fired.
  • The episodes of this serial are noticeably shorter than average. While it was normal at the time for individual episodes to fluctuate between twenty-three and twenty-five minutes, the episodes of this serial clocked in as little as just over twenty-one minutes. In addition, parts three and four begin with longer-than-usual reprises of the events leading up to the preceding cliffhangers, both clocking in at close to ninety seconds each. The final running time for the whole serial is 90 minutes, as is noted by the DVD of this story.
  • In his novelisation of the story, Terrance Dicks named the moon Delta III, but this was never used on-screen.
  • Tom Baker was credited as "The Doctor" in Radio Times, as opposed to the usual "Dr. Who". The use of this billing had previously occurred with Image of the Fendahl. In the closing titles, the credit "The Doctor" was not used until Castrovalva in 1982.
  • The sacrificial stone is referred to as the "Stone of Blood". An earlier story in this season was called The Stones of Blood.
  • The Doctor, talking to himself before his final encounter with Kroll, says he is seven hundred and sixty years old. This suggests he is either rounding up (as his age was given as seven hundred and fifty-nine in The Ribos Operation) or he's had a birthday since beginning the Key to Time mission. A birthday scene was originally planned for The Stones of Blood but dropped by Graham Williams, who considered it "too self-indulgent", as was noted in Doctor Who Magazine.
  • Romana is the only female character in this story. The previous occasion in which a situation like this occurred was three years previously in Pyramids of Mars in which Sarah was the only female character and would next occur in the Eleventh Doctor story Cold War in which Clara is the sole female.
  • According to the liner notes on the DVD, elements of this story were later reused in The Caves of Androzani. Both stories were written by Robert Holmes.
  • Alan Browning was originally cast as Fenner, but had to drop out due to illness.[1]
  • Frederick Jaeger,  T. P. McKenna and Gary Watson were offered to the role of Ranquin.
  • John Leeson replaced Martin Jarvis as Dugeen. Michael Sheard was also offered the role.
  • Neil McCarthy replaced George Baker as Thawn.
  • In A History of the Universe and aHistory, this story is arbitrarily dated to 2878, as Kroll manifests "every couple of centuries" and this is his fourth manifestation, so its been at least eight hundred years since Delta Magna was colonised. A more concrete date was finally made in Diamond Dogs, which takes place in the 51st century and refers to Rohm-Dutt as being an active criminal.
  • Graham Williams later described the story as "tacky" and that it contained the worst effects shot of his tenure.
  • Mary Tamm described this as her worst filming experience, being stuck in the mud and miles away from anywhere.
  • The plight of the swampies was intended as an allegory for the plight of indigenious American people.
  • Delta Magna was originally named Gannymede.
  • Michael Hayes was originally set to direct, but he was assigned to The Androids of Tara instead.
  • Graham Williams was taken ill during production, so Anthony Read and John Nathan-Turner (then unit manager) assumed producing duties. They were asissted by David Maloney.
  • To turn the Swampie actors' skin green, make-up artist Kezia Dewinne had selected a German product which was striking on camera and resistant to water. Unfortunately, she had neglected to order the special solvent required to easily remove the make-up. Regular removers proved less than effective, meaning that the performers went back to their lodgings that night and found themselves staining their bed sheets green. As such, arrangements were made for some of the actors to be sent to RAF Bentwaters at Woodbridge to take chemical showers; the remainder had their skin scoured by hand at their hotel. Despite these efforts, many of the Swampie artistes would sport a green tinge for weeks afterwards.
  • Visual effects designer Tony Harding intended to achieve the giant squid using a model which would be inserted into the filmed material via split-screen: the model would appear on the top of the frame and the location footage on the bottom. However, cameraman Martin Patmore, acting on poor advice, elected not to expose the upper portion of the film. This meant that a hard line would appear in the completed effect, rendering the join between the model work and the location footage regrettably obvious.


  • Part one - 6.5 million viewers
  • Part two - 12.4 million viewers
  • Part three - 8.9 million viewers
  • Part four - 9.9 million viewers


  • As reported by Doctor Who Magazine, this story was a replacement for one entitled The Lords of Misrule by distinguished screen writer Ted Willis. (Ted Willis never worked on Doctor Who. Thriller writer Lewis, who was best known for the seminal Get Carter, did work on an ultimately unused and untitled Key to Time story.)
  • John Leeson was cast in the role of Dugeen for contractual reasons, owing to his requirement to appear in a certain number of episodes in the season (John Leeson was in fact only cast as Dugeen at the very last minute, since Martin Jarvis - who was originally slated to play the role - dropped out. Additionally, there is no evidence that Leeson had such a requirement in his contract, given that K9 would often either fail to appear or have a non-speaking part in individual episodes or full serials throughout his tenure.)

Filming locations[]

Production errors[]

If you'd like to talk about narrative problems with this story — like plot holes and things that seem to contradict other stories — please go to this episode's discontinuity discussion.
  • The Doctor's finger movements while playing the reed pipe don't match the music.
  • The wall of the rocket silo is flimsy enough to wobble at the tap of a hammer.
  • The Doctor whips his galoshes on and off all through the story.
  • Neil McCarthy trips up on a line in part three. He begins to say "In which case the pr-" but corrects himself to "the Swampies most certainly do have some problems."
  • After Dugeen dies, his body can be seen in part four moving just for a second.
  • During Kroll's first appearance, two Swampie extras stand too close to the split-screen line used to insert the footage of the Kroll model, resulting in the tops of their heads disappearing.
  • At 1:13:12, a boom microphone briefly appears in the top right corner of the screen.
  • Several instances of Kroll grabbing its victims with its tentacles conspicuously have the footage reversed, most notably in Harg's death scene, where the leaking methane is flowing into the broken pipe rather than out of it.


  • While marooned in the TARDIS, K9 spent time playing games with the TARDIS computer. (AUDIO: Luna Romana)

Home video releases[]

DVD releases[]

Special Features (2007/2009 version)[]

  • Commentary by Tom Baker (The Doctor) and John Leeson (Dugeen) (carried over from the 2002 set)
  • Variations - A BBC local news programme visits the story location during filming. Includes interviews with Tom Baker and Mary Tamm
  • In Studio - A fascinating look inside one evening's studio recording of the story, recovered from time-coded monochrome production recordings. Mainly focuses on scenes involving Neil McCarthy, Philip Madoc and John Leeson, plus on-set discussions between Tom Baker, Mary Tamm and the director over how to play one scene.
  • There's Something About Mary... - Actress Mary Tamm looks back on her time as Romana
  • Philip Madoc - A Villain for all Seasons - A retrospective on actor Philip Madoc's numerous roles as a Doctor Who villain down the years
  • Continuities - Off-air continuity links from the story's original BBC1 transmission
  • Radio Times Billings - Original listings from Radio Times (DVD-ROM PC/Mac)
  • Coming Soon Trailer - Planet of Evil (2007 UK and Australian release only)
  • Photo Gallery
  • Production Subtitles


The Key to Time boxed set covers[]

Video releases[]

  • Released on VHS in the UK in June 1995
  • Released in 1995 in Australia and the US

Digital releases[]

  • The story is available for streaming in Canada, the US and UK through BritBox.
  • It is also available to download through iTunes.

External links[]