The Time Lords send the Doctor and Jo to the planet Solos in the dying days of Earth's decadent intergalactic Empire. An Earth delegation, led by the maniacal Marshal, has arrived on Solos, seeking to convert its atmosphere into one more suitable for habitation by humans. The native Solonians oppose this, but at the same time they discover that a crippling plague has befallen their people, turning them into hideously mutated monsters.

The Bristol Boys, Bob Baker and Dave Martin, had earned their first Doctor Who scriptwriting credit with Season Eight's The Claws Of Axos, and continued to submit ideas to the production office thereafter. One of these dealt with the colonial oppression of natives on an alien planet, inspired by Martin's concerns about South Africa's segregationist Apartheid policy. This caught the eye of script editor Terrance Dicks, who wanted to do a story about the British Empire's nineteenth-century colonialist ways. 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 introduced in his unused 1966 Doctor Who submission, “The Mutant”.

Dicks elected to take a patient approach to the new storyline, which was given the title “Independence”. He, Baker and Martin developed the idea over the course of several weeks, following which Dicks commissioned the first episode on May 20th, 1971. After this was submitted to the production office, Dicks and Letts suggested a variety of changes to the storyline for part two, such as the introduction of Jaeger (named after actor Frederick Jaeger, for whom the Bristol Boys had written on The Pretenders) and having the Doctor sabotage the Skybase power supply (rather than its malfunction occurring naturally due to a lightning strike, as originally conceived). The script for this installment was then commissioned from Baker and Martin on August 4th.

Part three was requested on September 3rd, with the final three episodes following on October 1st. Dicks and Letts continued to massage the storyline throughout this process, notably asking that a subplot about cloning be removed from the latter portion of the adventure, as it was felt to be overly complex. To effect Ky's transformation, the Doctor initially had to turn the crystal into a liquid form and inject it into Ky's neck; as scripted, the Solonians' final mutated form was an iridescent globe. Dicks also elected to change the slang term for the Solonians from “Munts” to “Mutts”, as the former was an authentic derogatory term used amongst the white settlers of South Africa in reference to the indigenous black population.

As production neared, the title for the story -- allocated the code Serial NNN -- became “The Emergents” and then The Mutants. Christopher Barry was appointed as director, having most recently helmed The Daemons at the end of the previous season. Barry was unhappy with the political bent of The Mutants and opted to emphasise the serial's science-fiction trappings, to the disappointment of Baker and Martin. Barry also elected to cast a black actor, Rick James, as Cotton. Although this made The Mutants unusually multiracial compared to other Doctor Who stories of its time, Baker and Martin were surprised to discover that Barry had not altered Cotton's scripted Cockney speech patterns.

Location work for The Mutants took cast and crew to Kent for six days in February 1972. The 7th and 8th were spent at Western Quarry in Northfleet, which served as the surface of Solos; while there, Manning reinjured the ankle she had twisted while recording Terror Of The Autons the year before. The next two days featured shooting around caves on Stone House Farm in Frindsbury, for material at the mouth of the Solonian caves. The cavern interiors were filmed on the 11th and 12th, at Chiselhurst Chalk Caves in Chiselhurst.

Studio taping then began on February 28th and 29th; The Mutants followed the usual Doctor Who pattern of recording every second Monday and Tuesday. The first session took place in BBC Television Centre Studio 4, and covered all the material for parts one and two. The second block, on March 13th and 14th, saw the production relocate to TC8. The first of these days was beset with problems. In contravention of BBC regulations, the Marshal's office had been erected over the studio fire line and had to be set up anew. Other sets and props were either incomplete, broken or missing altogether, drawing the wrath of designer Jeremy Bear. Nonetheless, the day's work -- episode three and scenes set at the hut and the clearing for parts four and five -- was completed with only a minimal overrun.

The remainder of the fourth installment was then recorded on the 14th. Two weeks later, the final studio session was held in TC3. March 27th saw episode five completed alongside material in the radiation chamber, the fuel store and the Skybase corridors for part six. The rest of the final installment was wrapped up on the 28th.

• 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 #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.