Romana is recalled to Gallifrey, but en route the TARDIS is drawn through a Charged Vacuum Emboitment into another universe, called E-Space. Landing on the planet Alzarius, the Doctor meets a group of humans who are trying to rebuild their spacecraft -- which crashlanded generations ago -- so they can return to their native Terradon. When Marshmen begin rising from the swamps during the dreaded time of Mistfall, however, the Doctor realises that there is something amiss on Alzarius, and begins to unravel a genetic riddle which stretches back centuries.

Andrew Smith was a teenaged Doctor Who fan who was eager to establish a career as a writer. He had submitted material to comedy programmes such as Not The Nine O'Clock News, and in mid-1978 had offered a storyline called “The Secret Of Cassius” to the Doctor Who production office. This was rejected by script editor Anthony Read, but Read was sufficiently impressed with Smith's potential that he encouraged the young man to continue sending in ideas.

When new Doctor Who script editor Christopher H Bidmead began reviewing the programme's files in early 1980, his attention was drawn to another of Smith's contributions, entitled “The Planet That Slept”. Bidmead was urgently seeking scripts for Doctor Who's eighteenth season, and he and producer John Nathan-Turner both wanted to encourage writers new to the show. Despite Smith's youth and inexperience, Bidmead commissioned him to write the first episode of “The Planet That Slept” on February 25th.

In Smith's original outline, the adventure involved the TARDIS landing on the planet Alzarius, where monstrous Marshmen rise from the swamps during the time of Mistfall. As the Doctor and Romana explore, a space freighter crashlands on Alzarius, and the time travellers must help its crew repair the ship while fending off the Marshmen and gigantic cave-dwelling spiders. They are aided by a young Marshchild, who has been rejected by the rest of her kind due to her pacifist nature. In the end, the Marshchild sacrifices herself to keep the Doctor and Romana safe until the mists vanish and the Marshmen retreat to the swamps.

Bidmead was impressed with the striking images that Smith conjured in “The Planet That Slept”, but asked him to make several changes as he fleshed the story out into script form. Bidmead wanted to give Doctor Who a much stronger scientific underpinning, and so he and Smith decided to make evolution an underlying theme of “The Planet That Slept”. The Marshmen, spiders and even the ship's crew would all be different evolutionary stages of the same creature. The space freighter had now become a passenger liner which crashlanded on Alzarius centuries before the start of the story.

Smith also had to integrate elements of a story arc that Bidmead had conceived, which would begin in “The Planet That Slept” and run through the next two serials. This had been developed at the suggestion of fan advisor Ian Levine, who recalled the tighter continuity between stories sometimes seen in Doctor Who during the Sixties. For his part, Bidmead wanted to impart more meaning to the Doctor's seemingly aimless wandering. Nathan-Turner was less keen on this idea, having witnessed the difficulties that overarching plots had imposed on Season Sixteen, when every story involved the search for a segment of the Key To Time. However, he ultimately consented to Bidmead's notion of a shorter story arc; this concerned the TARDIS becoming trapped in a pocket universe called E-Space, from which the Doctor would strive to escape.

Most significantly, however, Smith was asked to introduce a new companion devised by Nathan-Turner, called Adric. The character had already been written into State Of Decay, which would be made before “The Planet That Slept” but broadcast afterwards. It was decided that Adric would take over some of the Marshchild's role in the story. Smith also had to include Adric's brother, who had been called Afrus in Adric's original character description, but was now renamed Varsh.

Bidmead worked closely with Smith in preparing the script for the opening installment of “The Planet That Slept”, and on March 31st the remainder of the story was commissioned. It was now known that Smith's adventure would be the fourth story into production (and so was designated Serial 5R) but the third to be screened, after Meglos. Various changes were made during the scripts' development. A power struggle amongst the Outlers between Varsh and Tylos was eliminated, while a Marshchild character was reintroduced, replacing a mature Marshwoman. Lexeter was renamed Dexeter (to avoid confusion with Lexa in Meglos) while Terradon was initially spelt “Teradon”. Finally, in mid-June, Bidmead retitled the serial Full Circle.

Although State Of Decay -- the middle part of the E-Space trilogy -- had already been recorded, it was not until June 12th that Bidmead issued a document detailing the story arc. His idea was that the TARDIS would pass through a Charged Vacuum Emboitment (or CVE), a region of space-time in which the interaction of matter and antimatter particles with strong electromagnetic fields has created the doorway to a number of tiny subsidiary universes. The Doctor and Romana would then become trapped in such an “exo-space/time continuum” (dubbed “E-Space”, while the normal universe was termed “N-Space”) which would register on the TARDIS controls as having negative coordinates. Because E-Space would be very small -- containing just two galaxies -- the Doctor would have an unprecedented ability to steer the TARDIS. In addition to forming the background for the three E-Space stories, Bidmead planned that these details would also presage elements of the season's concluding adventure.

The director assigned to Full Circle was Peter Grimwade, who had previously worked on Doctor Who as a production assistant dating back to 1970's Spearhead From Space; he had come to know Nathan-Turner when both were involved with All Creatures Great And Small. Grimwade had recently completed the BBC's internal directors course, and had already worked on an episode of The Omega Factor. He was also a writer, having contributed to Z Cars, and had submitted an idea called “Zanadin” (later Time-Flight) to the Doctor Who production office which was being considered for the season's final slot.

Work on Full Circle began with three days on location at Black Park in Fulmer, Buckinghamshire, from July 23rd to 25th. Waterhouse rejoined the cast, having not appeared in Meglos. It was only at this stage that the Alzarians' ability to rapidly heal injuries was introduced; previously, Adric's knee had been mended by a spray that the Doctor provides. Smith visited the shoot on the first day, but was unwell and vomited on the Marshmen costumes.

The first studio block for Full Circle occurred on August 7th and 8th in BBC Television Centre Studio 3. The opening day dealt with all of the interior TARDIS sequences, while the second day was concerned with material in the Starliner lower decks and corridors, as well as the cave. The second recording session took place from August 21st to 23rd in TC6. The rest of the cave scenes were taped on the 21st, along with those in the Science Unit. The next day, model sequences were shot alongside material in the Great Book Room. The baby Terradonian was played by Alys Dyer, the daughter of production unit manager Angela Smith; she had also been the infant Pangol in The Leisure Hive earlier that year. Finally, scenes in the boarding area and corridors of the Starliner were completed on the 23rd.

Additional dialogue recording during postproduction on Full Circle included a mention of Citizen Darchir on the Starliner tannoy. This was an anagram of “Richard”, referring to Smith's friend Richard Walter; like Smith, Walter was a member of the Doctor Who Appreciation Society, and edited its fanzine, TARDIS. Buoyed by the success of Full Circle, Smith submitted several further ideas to the Doctor Who production office. These included “The Torson Triumvirate” later in 1980, and “The First Sontarans” in 1984. However, none of Smith's ideas were taken forward, and Full Circle wound up being his only contribution to the series. His subsequent credits included comedy sketch programmes on both television and radio.

Meanwhile, around the time that Full Circle was in production, it became clear that Tom Baker would not be returning to Doctor Who for an eighth season. Baker was already the longest-serving Doctor by two years, and was becoming visibly tired of the job; he also disagreed with several of the changes that Nathan-Turner had made to the series, and did not get along well with Waterhouse. Baker had hinted about leaving Doctor Who several times in the previous few years, but it was now clear that both the actor and the production team saw eye to eye on the situation. The final straw came when Nathan-Turner suggested that Baker would not be in line for a pay raise for Season Eighteen. With Baker's imminent departure becoming more and more certain, Nathan-Turner and Bidmead found themselves planning for a genuine end of an era...

• Doctor Who: The Handbook: The Fourth Doctor by David J Howe, Mark Stammers and Stephen James Walker (1992), Virgin Publishing, ISBN 0 426 20369 8.
• Doctor Who: The Eighties by David J Howe, Mark Stammers and Stephen James Walker (1996), Virgin Publishing, ISBN 1 85227 680 0.
• Doctor Who Magazine #327, 5th March 2003, “Archive: Full Circle” by Andrew Pixley, Panini Publishing Ltd.
• Doctor Who Magazine Special Edition #9, 22nd December 2004, “Another One Bites The Dust” by Andrew Pixley, Panini Publishing Ltd.
• In-Vision #48, January 1994, “Production” edited by Justin Richards and Peter Anghelides, Cybermark Services.