In the thirtieth century, Earth's decadent intergalactic Empire is in its dying days, but the cruel Marshal clings to power over the planet Solos. The Doctor and Jo arrive, tasked by the Time Lords with delivering a mysterious message. They learn that unrest on Solos is mounting as the Marshal tightens his grip, in defiance of orders from Earth to grant its people their independence. At his behest, the scientist Jaeger is conducting experiments to make the planet's atmosphere more suitable for humans. But the Solonians blame these trials for a virulent mutation, which is turning them all into hideous monsters.

Bob Baker and Dave Martin earned their first Doctor Who scriptwriting credit with 1971's The Claws Of Axos, and subsequently submitted further ideas to the production office. One dealt with the oppression of natives on an alien planet, inspired by Martin's concerns about South Africa's segregationist Apartheid policy. This aligned with script editor Terrance Dicks' interest in pursuing a narrative about British colonialism, and especially its twentieth-century demise with the independence of India and other territories. Producer Barry Letts suggested that the storyline could incorporate the concept of an alien species which evolved in stages like a butterfly, something he had explored in his unused 1966 submission, “The Mutant”. Martin was already thinking about a metamorphosing lifeform, having been inspired by discussions with a neighbour who was a biologist.

Dicks had had to work closely with Baker and Martin on The Claws Of Axos due to their relative inexperience in television. As such, he elected to take a patient approach with the new storyline, which was given the title “Independence”. He, Baker and Martin developed the narrative over the course of several weeks in early 1971 before the script for Episode One was commissioned on May 20th. After its submission, Dicks and Letts suggested a variety of changes to Baker and Martin's plans for Episode Two. These included the introduction of Professor Jaeger -- named after actor Frederick Jaeger, for whom Baker and Martin had written on The Pretenders -- and the Skybase power supply being sabotaged by the Doctor, rather than malfunctioning as a result of a lightning strike. The script for Episode Two was then commissioned on August 4th.

A subplot about cloning was removed from the later stages of The Mutants due to its complexity

Episode Three was requested on September 3rd, by which time the title had become “The Emergents”. The final three parts were commissioned on October 1st as The Mutants. Dicks and Letts continued to massage the storyline throughout this process, notably asking that a subplot about cloning be removed from the later stages of the adventure, due to its complexity. To effect Ky's transformation, the Doctor initially had to turn the crystal into a liquid form, which was then injected into the Solonian's neck; as scripted, Ky's final mutated form was an iridescent globe. Dicks also changed the slang term for the Solonians from Munt to Mutt. The former was intended to be a contraction of the phrase “mutant native”, and was a genuine derogatory term used by the white settlers of South Africa to refer to the indigenous black population. However, Head of Serials Ronnie Marsh feared that it was too easily mistaken for a more familiar vulgarity.

Christopher Barry was appointed to direct The Mutants, having most recently worked on The Daemons at the end of the previous season. Barry was unhappy with the political bent of the scripts and opted to emphasise their science-fiction trappings, to the disappointment of Baker and Martin. It was Barry's decision to cast a black actor, Rick James, as Cotton. Although this made The Mutants unusually multiracial compared to other Doctor Who stories of the era, Baker and Martin were surprised to discover that Cotton's scripted Cockney speech patterns had not been altered, given that James hailed from Antigua and Barbuda.

Location work for The Mutants took the cast and crew to Kent for six days in February 1972. The 7th and 8th were spent at the disused Western Quarry in Greenhithe, which served as the surface of Solos. There, Katy Manning reinjured the ankle she had twisted while recording Terror Of The Autons the year before. Meanwhile, production assistant Fiona Cumming became unwell and fell asleep in her room without placing the unit's cash on hand in the hotel vault. This proved serendipitous, as Cumming awoke the next morning to discover that the hotel porter had looted the vault and disappeared!

On February 9th and 10th, material at the cavemouth on Solos was filmed at Stone House Farm near Frindsbury. Part of the 10th was also spent at the man-made Chislehurst Caves in Chislehurst, the setting for scenes within the caverns. Work there continued on February 11th and 12th. Barry had originally wanted to film this material at Dancing Ledge near Langton Matravers, Dorset, but safety concerns forced a change of plans. The Chislehurst shoot coincided with the onset of industrial action across Britain, which caused sporadic power outages. Barry's team was caught in one of these blackouts, forcing them to navigate Chislehurst Caves in the pitch dark.

Studio recording for The Mutants followed the season's normal pattern, with two-day blocks occurring every second Monday and Tuesday. The first session, on February 28th and 29th, took place at BBC Television Centre Studio 4 in White City, London. Episode One was recorded on the Monday, and Episode Two on the Tuesday. Garrick Hagon, who played Ky, was pressed into additional service as the voice of the Skybase public address system, heard in these and other installments.

The second block, on March 13th and 14th, saw the production relocate to TC8. Unfortunately, the Monday was beset with problems. The set for the Marshal's office had been erected over the studio fire line, meaning that it had to be struck and re-erected. Other sets and props were either incomplete, broken or missing altogether. Nonetheless, Barry was able to complete the day's schedule -- most of Episode Three and scenes set at the hut and the clearing for Episodes Four and Five -- with only a minimal overrun. The remainder of Episode Four was then recorded on the 14th, alongside the Episode Three sequence of Jo glimpsing Sondergaard in the caves.

The Mutants moved to TC3 for the final studio session, on March 27th and 28th. Episode Five was completed on the Monday, as was the material set in and around the radiation chamber and the fuel store for Episode Six. In Episode Five it was John Hollis' turn to play a voice-only part, in addition to his main role as Sondergaard, as he assumed an Australian accent to read the Hyperion pilot's dialogue. The rest of the concluding installment was then recorded on the 28th. On this day, Ky's attack on the Marshal challenged Barry with one of the most complicated set-ups in Doctor Who's history, requiring the combination of no fewer than six inputs to produce the final footage.

With the broadcast of The Mutants Episode One on April 8th, Doctor Who now led into The Wonderful World Of Disney, which pushed the High Adventure movie strand later in the evening. On April 22nd, the day that Episode Three aired, Doctor Who also gained a new lead-in, with The Basil Brush Show replacing We Want To Sing; the intervening news update remained. The Wonderful World Of Disney did not air on April 29th; that night's quarterfinal of the European Football Championship, featuring England versus West Germany, instead pushed High Adventure back into its old timeslot, immediately after The Mutants Episode Four. The following week, The Basil Brush Show was preempted by the FA Cup Final between Arsenal and Leeds United, with the shorter The Pink Panther taking its place prior to the news and weather, and then the penultimate installment of The Mutants.

• Doctor Who Magazine #230, 27th September 1995, “Archive: The Mutants” by Andrew Pixley, Marvel Comics UK Ltd.
• Doctor Who Magazine Special Edition #2, 5th September 2002, “Family Affair” by Andrew Pixley, Panini Publishing Ltd.
• Doctor Who: The Complete History #18, 2018, “Story 63: The Mutants”, edited by John Ainsworth, Hachette Partworks Ltd.
• Doctor Who: The Handbook: The Third Doctor by David J Howe and Stephen James Walker (1996), Virgin Publishing.
• Doctor Who: The Seventies by David J Howe, Mark Stammers and Stephen James Walker (1994), Virgin Publishing.