The novelistation of 'The Mutants' was written by Terrance Dicks.
Back Cover Blurb
A massive shape scuttles out of the darkness and strikes the Overlord down. It is a Mutant - a huge insectoid creature. It moves in a crouch, its back arched and scaly, with huge knobbly vertebrae. The controlling Overlord, the Marshal, has ordered all such Mutants killed instantly.
What was happening to the people of Solos? Why are they gradually turning into Monsters? hands that become claws, flesh that turns scale-like...
When Doctor Who meets the Marshal and Jaefer, he realizes that all is not as it appears to be. The Marshal has a sinister plan to gain control of this planet, and Doctor Who must save Solos from this mad earthman, as well as save the Solonians theselves.