This story begins with a death sentence to be carried out on an alien found guilty of a most treacherous act which is not carried out exactly according to plan.
Switch to the quarry, a bit of an in-joke after all of the Pertwee years where the quarry seemed to become an almost required set. It is ironic too that the alien story element is actually a silicon based lifeform!
In the last Sarah Jane Smith story, I believe that she was the longest serving of the Doctor's companions, the story involves regeneration of an alien life-form through the consumption of radiation from a nuclear power station despite military intervention which, as always, is ineffective. The story is reminiscent of the old horror movie of the hand which has a life of it's own but in a different way. The alien life form when recreated has a strange beauty and a believable tale to tell convincing the Doctor to return it to it's own planet.
Ashes to ashes, dust to dust, the usual story of treachery and deceit is relived again with an interesting transformation of form once again and a twist in the tale.
This story is not apparently as dark a tale as the others under Philip Hinchcliffe's excellent production. On closer inspection however, this is a story of genocide, but it is not really explored sufficiently. It is also like the Egyptian theme of 'Pyramids of Mars' and the story of Horus and Osiris. A stunning contrast to the 'Masque of Mandragora' which precedded it and to the 'Deadly Assassin' which followed it, this is certainly an interesting story.
Just one thing, it always irritated me about Elisabeth Sladen wearing those Andy Pandy overall things.