The Doctor's old friend Zastor summons him back to the planet Tigella, where the population is divided between the zealous Deons and the scientific Savants. Something is going wrong with the Dodecahedron, Tigella's main power source, but the Deons have forbidden the Savants from investigating. Meanwhile, on the nearby world of Zolfa-Thura, a group of mercenaries revives the ruined planet's last survivor: the cactus-like Meglos. The shapeshifting Meglos steals the Doctor's form and traps the TARDIS in a time loop. He then travels to Tigella, where his goal is to steal the Dodecahedron -- and frame the Doctor for the crime.

When Christopher H Bidmead became Doctor Who's script editor at the start of 1980, he found himself scrambling to develop stories for the programme's upcoming eighteenth season. Both he and producer John Nathan-Turner were eager to bring new talent onto the show, and so one of the first people Bidmead contacted was an actor he knew named Andrew McCulloch. McCulloch had formed a writing partnership with a fellow thespian, John Flanagan, although their only televised work to date was an unsuccessful sitcom pilot called Bricks Without Straw. In early February, Flanagan and McCulloch developed a story idea focussing on an exaggerated schism between science and religion, which was exploited by the villainous Meglos -- whose true nature was inspired by a potted cactus sitting on McCulloch's kitchen table.

On February 25th, Flanagan and McCulloch were commissioned to expand Meglos into a scene breakdown. Nathan-Turner was unimpressed with an adventure that he saw as just standard Doctor Who fare, and Bidmead felt that the writers were expending too little effort on the plot and too much on the eponymous villain. Nonetheless, with little time to seek an alternative, full scripts were requested on March 10th. Since Flanagan and McCulloch wrote quickly, it was decided that “The Golden Star” -- as the adventure had now been renamed -- would be the third story into production, coming after The Leisure Hive and State Of Decay. Unlike the latter, “The Golden Star” would not feature new companion Adric, and so it would be the year's second broadcast adventure, coming immediately before his introduction in Full Circle.

The name Brotadac was jokingly devised as an anagram of “bad actor”

During the writing process, a number of working titles were considered -- apparently including “The Golden Pentangle”, “The Golden Pentagram” and “The Last Sol-Fataran” -- before the serial became “The Last Zolfa-Thuran” by the end of April. Several of these names reflected the evolving shape of the Dodecahedron: it was originally a five-sided object, thereby influencing the number of screens found on Zolfa-Thura, rather than a twelve-sided object made up of pentagons. Keen to inject more science into Flanagan and McCulloch's scripts, Bidmead suggested the inclusion of the chronic hysteresis, deriving the terminology from a loop-like phenomenon encountered in disciplines such as electromagnetism. Meanwhile, Flanagan and McCulloch jokingly devised the name Brotadac as an anagram of “bad actor”!

As with Peter Moffatt on State Of Decay, Nathan-Turner chose a former colleague from All Creatures Great And Small to direct “The Last Zolfa-Thuran”. This was Terence Dudley, whose friend Alvin Rakoff was married to Jacqueline Hill, one of the original stars of Doctor Who. Hill had played Barbara Wright between 1963 and 1965, and had subsequently paused her acting career to raise the couple's children. She was now eager to return to work, and Dudley offered to cast her as Lexa in “The Last Zolfa-Thuran” -- much to the approval of Nathan-Turner, who was keen to appeal to the show's longtime fans. In Flanagan and McCulloch's original storyline, Lexa had simply faded from the narrative once she ceased contributing to the plot. Bidmead suggested her act of self-sacrifice as a more dramatic end for the character.

One of Nathan-Turner's earliest decisions as producer had been to drop both Romana and K·9 from Doctor Who partway through Season Eighteen. Although Lalla Ward had already revealed her imminent departure to the British press in May, it took until June 7th for word of K·9's upcoming exit to leak out. It was The Sun that initially revealed the news and, despite being provided some vaguely-worded denials by Nathan-Turner, the paper seized on the opportunity to drum up publicity and outrage by launching a “Save K·9” letter-writing campaign.

Although Flanagan and McCulloch had hoped that many of the scenes on Zolfa-Thura and in the Tigellan jungle would be achieved on location, Nathan-Turner decided that their scripts represented an opportunity to save money by confining all recording to the studio. For a time, it was suggested that the Gaztaks might exhibit an East Asian appearance, with Bill Fraser and Frederick Treves (as Grugger and Brotadac) made up accordingly, but this notion was dropped. Ward's costume for “The Last Zolfa-Thuran” was inspired by Frances Hodgson Burnett's 1886 children's novel Little Lord Fauntleroy. Meanwhile, Bidmead was required to significantly expand some of the scripts -- particularly Episode Three, which was found to be running seven minutes short. Amongst the added material was Romana leading the Gaztaks in a circle through the jungle.

Meglos suited John Nathan-Turner's preference for more varied and unusual story titles

The serial's first studio session was held from June 25th to 27th at BBC Television Centre Studio 8 in White City, London. Rather than taking the usual approach of recording set-by-set, Dudley opted to mix and match material from all of the Tigellan locales on each of the three days. Tom Baker was now slowly recovering from the illness which had plagued him for weeks, but his long convalescence was still dampening his mood, and he disliked the latex facial appliance he had to wear to give him a cactus-like appearance. By this stage, the adventure was once again known by the writers' original choice: Meglos. This suited Nathan-Turner's preference for more varied and unusual story titles in Doctor Who.

The second studio block took place in TC3 and spanned July 10th to 12th, with Dudley now primarily concentrating on the Zolfa-Thuran sequences. The TARDIS scenes were also recorded on the first day, modelwork was completed on the second day, and the last day included material aboard the Gaztak spaceship. A major element of the work scheduled for July 11th was the use of a new process called Scene-Sync, which was a more advanced version of the chroma key technique used on Doctor Who since 1969. Chroma key relied on two cameras, with the output of one camera electronically replacing a “key” colour in the output of the other. Unfortunately, the effect was often spoilt if the cameras had to move, since it was almost impossible to do so synchronously. Scene-Sync remedied this issue by electronically slaving one camera to the other. It was an unproven process at the BBC, and so it was offered to Doctor Who for use on Meglos as a test case, free of charge. Dudley's implementation of Scene-Sync for some of the Zolfa-Thuran sequences was judged to be a success and, as a result, the technology became more widely adopted.

The broadcast of Meglos coincided with the BBC's announcement on October 7th that K·9 would indeed be leaving Doctor Who after all. Like The Leisure Hive, the serial's first two episodes were scheduled at 6.15pm, bookended by The Dukes Of Hazzard (plus the news) and Larry Grayson's Generation Game. With strong opposition from Buck Rogers In The 25th Century on ITV, ratings continued to flounder. As such, with Episode Three on October 11th, the BBC shook up its Saturday evening schedule. Doctor Who was brought forward to 5.40pm, and The Basil Brush Show returned as its lead-in. Larry Grayson still followed the Doctor, but now with a news update in-between. Despite Bidmead's efforts, all four episodes of Meglos were very short, with Episode Four running for less than twenty minutes. As a result, it was scheduled in a shorter timeslot, starting at 5.45pm.

Meanwhile, on August 29th, popular waxworks museum Madame Tussauds had unveiled a display called The Doctor Who Experience to celebrate the debut of the programme's eighteenth season the next day. The exhibit included a waxwork of Tom Baker, in costume as the Fourth Doctor. Following the broadcast of Meglos, the display gained a statue of the Doctor's Meglos doppelganger. Consequently, Baker became the first person to be represented at Madame Tussauds by two different waxworks at the same time.

• Doctor Who Magazine #285, 15th December 1999, “Archive: Meglos” by Andrew Pixley, Marvel Comics UK Ltd.
• Doctor Who Magazine Special Edition #9, 22nd December 2004, “Another One Bites The Dust” by Andrew Pixley, Panini Publishing Ltd.
• Doctor Who: The Complete History #32, 2017, “Story 110: Meglos”, edited by Mark Wright, Hachette Partworks Ltd.
• Doctor Who: The Eighties by David J Howe, Mark Stammers and Stephen James Walker (1996), Virgin Publishing.
• Doctor Who: The Handbook: The Fourth Doctor by David J Howe, Mark Stammers and Stephen James Walker (1992), Virgin Publishing.
• In·Vision #47, November 1993, “Production” edited by Justin Richards and Peter Anghelides, Cybermark Services.