Waste from a chemical plant, Global Chemicals, in Wales has mutated the local insects, turning them into deadly giant monsters. With the aid of a team of local ecologists, the Doctor and Jo set about stopping the monsters and the environmental destruction being wrought by the refinery. But when Mike Yates goes in undercover, he discovers that Global Chemicals' director is not just unscrupulous: he's inhuman.

Early in 1972, Doctor Who producer Barry Letts and script editor Terrance Dicks were becoming increasingly concerned with environmental issues. Letts had read an article about the dangers of pollution in the magazine The Ecologist, and he and Dicks felt Doctor Who offered them an opportunity to tell a positive message about protecting the environment. To this end, Letts again sought the services of his friend Robert Sloman. Sloman and Letts had cowritten The Daemons two years earlier, and Letts had been heavily involved with Sloman's second serial, The Time Monster, the following season.

Sloman was working on a storyline for Doctor Who by October, and The Green Death was commissioned on November 30th. As with his two previous contributions to the series, this would be the final story of the season. By this point, Katy Manning had suggested to Letts that she might leave Doctor Who at the end of Season Ten. Letts consented by January 1973: he agreed that spending longer than three years on the programme could be injurious to Manning's career. The producer also suspected that Jon Pertwee might be departing after the eleventh season, and he did not want his two stars quitting Doctor Who simultaneously. Sloman was therefore asked to amend The Green Death to include Jo's decision to marry Cliff Jones, who would be written as a younger version of the Doctor -- the only type of person the production team could envision Jo leaving UNIT for.

Sloman's scripts also went through a variety of other, more minor modifications. The chemical company was originally called Universal Chemicals, and then United Chemicals; however, because such a business actually existed, it was later changed to Global Chemicals. Similarly, the character of Charles Bell was renamed Ralph Fell to avoid confusion with a real-life chemical company executive. As well, a Nuthatch denizen nicknamed “Face” was excised (and the relevant dialogue assigned to Nancy instead). Wholeweal was inspired partly by Greenpeace and partly by the Biotechnic Research And Development (BRAD) community which had been established in Wales. It was briefly thought that Yates' role in the latter stages of the serial might be given to Benton instead, but this idea was not pursued. The two actors -- Richard Franklin and John Levene -- were contracted on March 6th and 8th, respectively. Nicholas Courtney, meanwhile, was belatedly booked to play the Brigadier on the 14th, during filming.

The director assigned to Serial TTT was Michael Briant, who had last worked on The Sea Devils the previous year. Amongst the suggestions made to Briant for the role of Clifford Jones was Stewart Bevan, then Manning's real-life boyfriend. Briant was reluctant to have Bevan audition, fearing the on-set ramifications -- especially considering how upset Pertwee had become over the prospect of Manning's departure -- but Bevan ultimately proved the only suitable choice for the role.

The location schedule for The Green Death was unusually hectic, and so Briant assembled a second unit for close-ups and establishing shots, directed by production assistant John Harris. Filming began on March 12th with scenes on the road and at Wholeweal; these took place around Troed-y-Rhiw Jestyn near Deri in Mid-Glamorgan. The second half of the day, as well as the next three days, were occupied by scenes in and around the mine. These were filmed at Ogilvie Colliery in Deri; the nearby Colliery Quarry also served as Metebelis 3 on the 13th. During the Ogilvie material, John Levene ad-libbed Benton's reference to two UNIT soldiers as “Dicks” and “Betts” in honour of the production team. Filming concluded with three days -- the 16th, 19th and 20th -- at the RCA International factory at Bryn-Mawr, Powys, which served as the Global Chemicals building. In between, model filming took place on March 17th.

Studio recording -- all of which took place in BBC Television Centre Studio 3 -- then began with a two-day session on Monday, April 2nd and Tuesday the 3rd for episodes one and two. Exactly two weeks later, the majority of parts three and four were taped on the 16th and 17th; the only exception was the material in the computer room from the fourth installment, which was held over until the final studio block. Deviating slightly from the Season Ten pattern, these last two days occurred on a Sunday and Monday, April 29th and 30th. Unfortunately, Tony Adams developed peritonitis prior to this final recording session, meaning that he could not continue as Elgin. However, Elgin's appearances in the latter third of The Green Death were minimal, and so he was replaced with a new character, Mr James, played by Roy Skelton.

The departure of Katy Manning on the 30th marked the beginning of the end for the so-called UNIT family, which had been together since the start of Season Eight two and a half years earlier. Her decision was announced to the public on June 22nd. Manning worked mainly in theatre after leaving Doctor Who, and in 1982 moved to Australia with her two children. She maintained her connection with Doctor Who, however, appearing in the video Lust In Space and providing the voice of erratic Time Lady Iris Wildthyme in several audio dramas from Big Finish Productions.

Despite the burgeoning behind-the-scenes instability, Doctor Who had enjoyed tremendous growth during Season Ten, which came to a close with the broadcast of The Green Death part six on June 23rd (although the tenth recording block would continue with The Time Warrior, the first serial of Season Eleven). The anniversary season had seen Doctor Who perform as well in the ratings as it ever had, even returning to the Top Ten programmes of the week during the telecast of The Three Doctors. After years of uncertainty, Doctor Who was enjoying renewed success, but its next big test was just around the corner...

• Doctor Who: The Handbook: The Third Doctor by David J Howe and Stephen James Walker (1996), Virgin Publishing, ISBN 0 426 20486 7.
• Doctor Who: The Seventies by David J Howe, Mark Stammers and Stephen James Walker (1994), Virgin Publishing, ISBN 1 85227 444 1.
• Doctor Who Magazine #320, 21st August 2002, “Archive: The Green Death” by Andrew Pixley, Panini Publishing Ltd.
• Doctor Who Magazine Special Edition #2, 5th September 2002, “Saturday Night's Alright For Fighting” by Andrew Pixley, Panini Publishing Ltd.