On the marshy third moon of Delta Magna, the Doctor and Romana obtain strange readings as they hunt for the fifth segment of the Key To Time. Their search is complicated by tensions between the primitive Swampies -- the native species of Delta Magna, long ago relocated to the moon by human colonists -- and the crew of a new refinery. Intent on war, the Swampies have sought the help of gun-runner Rohm-Dutt, unaware that he plans to supply them with faulty weaponry. As hostilities escalate, the Swampie god -- a gargantuan squid named Kroll -- rises from the marsh, to wreak untold destruction.

With Doctor Who's sixteenth season comprised of a story arc depicting the Doctor's quest for the Key To Time, script editor Anthony Read sought to finalise the year's six serials at an early stage. Despite his efforts, however, two problems persisted into May 1978: the fifth slot was still vacant, and it appeared that Ted Lewis would be unable to complete his scripts for the intended fourth adventure, “Shield Of Zarak”. Consequently, Read turned to two writers who had already contributed to Season Sixteen: Robert Holmes, whose The Ribos Operation would introduce the umbrella theme, and David Fisher, who had devised The Stones Of Blood.

On May 26th, both Holmes and Fisher were commissioned to provide their second serials of the year. Fisher's story, The Androids Of Tara, was now planned to be the season's penultimate adventure, while Holmes' offering, “Moon Of Death”, would replace “Shield Of Zarak”. Holmes was a Doctor Who veteran -- not to mention its former script editor -- and so Read was confident that he would be able to finish his scripts by the time director Michael Hayes joined the production in early June. Read asked Holmes to incorporate a monster whose size was unsurpassed in the annals of Doctor Who. He also advised the writer to temper the humour he usually injected into his scripts; in recent months, BBC Head of Serials Graeme MacDonald had repeatedly criticised overt comedy in Doctor Who.

Robert Holmes' narrative was inspired by the plight of the original peoples of the United States

Holmes developed a narrative -- now called “The Horror Of The Swamp” -- which was inspired by the plight of the original peoples of the United States. Having been driven off their traditional territories and onto specially-designated reservations to make way for settler populations, indigenous groups were increasingly confronting attempts by corporate and governmental interests to exploit their new lands as well. This inspired the establishment of the human-run refinery on the moon which had become home to the Swampies, the displaced original peoples of Delta Magna (originally, Gannymede). Unfortunately, the requirements imposed on Holmes by the production team -- together with the imperative of accommodating the Key To Time storyline -- contributed to “The Horror Of The Swamp” becoming an unsatisfying experience for the writer.

No more impressed was Hayes, who feared that it would be impossible to successfully realise Kroll -- Holmes' giant monster -- on the limited Doctor Who budget. Fortunately, Fisher had already started to deliver his scripts for The Androids Of Tara. In mid-June, producer Graham Williams agreed that The Androids Of Tara and “The Horror Of The Swamp” could switch places in both the production and broadcast schedules, with Hayes becoming responsible for Fisher's serial. The new director assigned to Holmes' story was Norman Stewart, who had handled another serial with intense effects requirements -- Underworld -- the year before. As the summer progressed, Holmes' scripts came to be known as The Power Of Kroll.

Stewart was forced to make a number of cast changes as his production dates loomed. He had originally approached Martin Jarvis to play Dugeen, but Jarvis dropped out in mid-August. Given the story's marshland setting, Holmes had avoided using K·9, since he knew that the prop would not be able to cope with such an environment. As such, it was decided that John Leeson -- who usually voiced the robot dog -- should instead portray Dugeen. This was formally approved on August 22nd.

Then, in early September, Fenner actor Alan Browning became unwell and it was decided that he would have to be replaced. Stewart contacted Philip Madoc -- who had played several villainous roles in Doctor Who over the years, dating back to The Krotons in 1968 -- but a misunderstanding led Madoc to believe that he was being offered the more substantial part of Thawn. The actor was therefore very disappointed to find himself relegated to a relatively minor role, and The Power Of Kroll proved to be Madoc's last involvement with Doctor Who.

At this time, a four-part Doctor Who story was typically allocated two studio sessions and a week of location filming. For The Power Of Kroll, however, Stewart arranged to sacrifice one of his studio blocks in favour of an extra week on location in Suffolk. The first day of filming was September 18th, at the Maltings in Snape. Around this time, Williams fell badly ill and, for the next several weeks, Read and production unit manager John Nathan-Turner assumed many of his duties. They were assisted by David Maloney, the producer of the BBC's other science-fiction drama, Blake's 7. He was very familiar with Doctor Who, having directed a number of serials since 1968, most recently The Talons Of Weng-Chiang in 1977.

September 19th took cast and crew to Iken Cliff at Iken, and was the first day to feature the Swampies. To turn the 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 shed 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 bedsheets. As such, arrangements were made for some of the actors to be despatched 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.

This was not the only mishap to befall The Power Of Kroll. 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.

The remaining work at the Maltings was completed on September 20th and 21st. Part of the latter day was spent back at Iken Cliff, which was also the venue on September 22nd. During the weekend, the Swampie settlement was constructed at Iken Cliff, and filming resumed there from September 25th to 28th; a reserve day on the 29th appears not to have been required.

The refinery model could not be photographed from the correct angle, due to the limitations of the water tank

From October 9th to 11th, production shifted to BBC Television Centre Studio 6 in White City, London. On the first day, most of the scenes in the refinery control centre and the adjacent corridor were completed. Sets in use on the 10th included those for the pump room, the rocket silo and the gantry. The remaining control centre and corridor sequences were recorded on the last studio day, together with those in the Swampie temple. Work on The Power Of Kroll then wrapped up at Bray Studios in Water Oakley, Berkshire, where model filming took place on October 19th and 20th. Effects problems continued to plague the serial, however, as Harding discovered that his refinery model could not be photographed from the correct angle, due to the limitations of the water tank.

For a long time, there was considerable uncertainty as to exactly when The Power Of Kroll would air. For the last three years, Doctor Who had been placed on a brief hiatus over the holiday season, and this was contemplated for Christmas 1978 as well. Ultimately, it was decided that no such pause should be incorporated into Season Sixteen, meaning that Episode One would air on December 23rd. Late in the year, however, the British television industry was rocked by the latest in a virtually annual succession of labour disputes. Not only did this delay the final dubbing of the last three episodes until scant days before they were broadcast, but it also threw the transmission schedule into doubt, as BBC1, BBC2 and ITV all experienced interruptions due to the strike action.

In the event, however, The Power Of Kroll did indeed debut on December 23rd. It was brought forward by ten minutes to 6.15pm, airing between Basil Brush's Magical Christmas and The Little And Large Christmas Show. The following week, the BBC's Saturday evening schedule was overhauled, with Jim'll Fix It occupying the slot before Doctor Who which, in turn, led into Saturday Night At The Movies. The season finale of The Basil Brush Show, broadcast earlier in the evening of December 30th, pushed Episode Two of The Power Of Kroll back to 6.30pm, but the last two installments returned to the programme's usual 6.25pm timeslot.

Meanwhile, with just one serial left to be made as part of Doctor Who's sixteenth recording block, behind-the-scenes tensions had begun to mount. Mary Tamm had become disenchanted with her character development, but refused to officially confirm her status for Season Seventeen. Anthony Read, on the other hand, was unambiguous in his decision to leave Doctor Who after a year and a half, to concentrate on his career in publishing. Read initially suggested Holmes as his replacement, only to learn that he had no interest in returning to his old job. Read then recommended Douglas Adams, who had written The Pirate Planet for Season Sixteen. Adams met with a convalescing Williams in early October and -- although he was contending with the burgeoning success of his radio play The Hitchhiker's Guide To The Galaxy -- he immediately agreed to the appointment.

Tom Baker felt that, after 5 years on Doctor Who, he should have more say in its creative direction

But the most tumultuous situation concerned Tom Baker. Whereas Baker had enjoyed an amicable working relationship with Williams' predecessor, Philip Hinchcliffe, matters were more strained between the series star and his current producer. Baker felt that, after five years on Doctor Who, he should have more say in the programme's creative direction and, around September, he had aired his grievances to MacDonald and the Controller of Programmes for BBC One, Bill Cotton. By October, Baker's demands had escalated to include approval of virtually every facet of the production, including scripts, directors and casting.

MacDonald considered calming the situation by moving Williams to another programme, but finally decided that it was time to cut ties with Baker. He instructed Williams to fire his lead actor, but the producer informed him that this was unnecessary: Baker had already pushed a note through his mail slot, declaring his intention to quit Doctor Who...

• Doctor Who Magazine #312, 9th January 2002, “Archive: The Power Of Kroll” by Andrew Pixley, Panini Publishing Ltd.
• Doctor Who Magazine Special Edition #9, 22nd December 2004, “I'll Put You Together Again” by Andrew Pixley, Panini Publishing Ltd.
• Doctor Who: The Complete History #30, 2016, “Story 102: The Power Of Kroll”, edited by Mark Wright, 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 #36, February 1992, “Production” edited by Justin Richards and Peter Anghelides, Cybermark Services.