This story is in essence very similar to the previous season's Colony in Space. Both feature two sides battling over an inhospitable planet apparently inhabited by a mysterious race of monsters whose origins are unknown. The difference in The Mutants is that the action is viewed mostly from the point of view of the oppressing humans rather than from that of the oppressed natives; but, as in Colony in Space, it takes the Doctor to figure out what is really going on.
The idea of the Doctor being stranded on Earth is, by this point, really starting to wear thin. The Time Lords have already been responsible (or so it is implied) for his trips to Uxarieus in Colony in Space and Peladon in The Curse of Peladon, but now we have them giving him a far more blatant mission: to deliver a message pod. One wonders, in passing, why they could not simply have materialised the pod in front of its intended recipient - but then of course there would have been no story.
As it is, the Doctor and Jo dash off to Solos, there to lock horns with the obsessive Marshal as he dedicates himself to 'cleansing' the planet. The Doctor's distaste for this imperialistic attitude is well brought across; and one of the nice things about the story is the way he manages to stay one step ahead of the Marshal and his scientist Jaeger all the way through, even when supposedly working in a forced alliance with them.
Some fine scripts by Bob Baker and Dave Martin are well served by Christopher Barry's characteristically polished direction. The mutants themselves are both well conceived and nicely designed; most people have a fear of insects, and the thought of the hapless Solonians transforming involuntarily and seemingly at random into giant ant-like creatures is quite terrifying - especially as they seem to be the 'good guys'.
The scenes set on the planet's surface and in the caves are particularly well shot and directed, making Solos one of the most effectively realised alien worlds ever presented in Doctor Who. 'There is a surprising amount of film in this story,' noted David Gibbs in Star Begotten Volume 3 Number 1/2, dated winter/spring 1989. 'All scenes set in the underground mines were filmed on location, as were the bulk of the planet exteriors (and even those in the studio - Varan's village - were melded in almost imperceptibly). For once a Doctor Who planet really does [look] like an alien world, helped in no small part by the copious amounts of the swirling mist that pervades every inch of the "Solonian" surface...'
One of the less praiseworthy aspects of the production on this occasion is the acting of the guest cast, none of whom is particularly impressive. Probably the best is Paul Whitsun-Jones as the single-minded Marshal, a worthy opponent for the Doctor who manages to make the act of threatening into an art form.
Undoubtedly the worst is Rick James as Cotton, who puts in a strong bid for the title of worst performance ever seen in Doctor Who. His woodenness and apparent inability to deliver his lines with any degree of conviction unfortunately tend to undermine every scene in which he appears, while Christopher Coll as his partner Stubbs struggles gamely to make the script work. Even the weakest elements in a story can have their admirers, however, and Dallas Jones took a rather different view in A Voyage Through 25 Years of Doctor Who, dated December 1988: 'The story had two characters I thoroughly enjoyed watching, namely Stubbs and Cotton... It was nice to see Cotton played by a black actor, especially as the story could be seen as commenting on the South African apartheid problem.'
Another aspect of the The Mutants that has attracted both condemnation and praise is the incidental music by Tristram Cary, with whom Christopher Barry had previously worked on the Daleks' debut story - also called, by a strange coincidence, The Mutants. Like Malcolm Clarke on The Sea Devils, Cary adopted an atonal musique concrete approach, creating an electronic landscape of beeps and whistles that, while admittedly quite in keeping with the story's themes and settings, is highly distracting.
Overall, though, The Mutants is a good story - and one that, with its strong anti-apartheid and anti-colonial messages, gives a good illustration of the increasingly moralistic tone that producer Barry Letts was bringing to the series at this time.