The Doctor, Sarah and Harry transmat down to the Earth from space station Nerva in order to make some repairs to the transmat receptor there before Vira begins the mass transportation of the human population. As the Doctor works on the problem the two humans decide to explore this distant future of their own world, stunned by the passing of all that they once considered part of their civilisation. Inevitably, Harry gets into trouble and the Doctor is captured by human colonial explorers, those who went out into space when Earth became uninhabitable. They have an Empire of their own, and are less than thrilled by the reappearance of the people of Mother Earth- especially as something on that planet has imprisoned them and is murdering them, one by one...
The Sontaran Experiment is a very brief story, and like most two parters, would have to do something rather awful to outstay its welcome. Fortunately, such criticisms are unnecessary. It fits its length perfectly, and manages to establish a heroic image for the fourth Doctor despite Tom Baker's enforced immobility for much of the second episode. Kevin Lindsay manages to make Styre a believably different individual from Commander Linx- far more the viciously sadistic interrogator than the supposedly noble warrior, and turns in another fine performance. The use of film works well with the location used, creating a very bleak look to the production, very much in keeping with the stark cruelty of the storyline. Production is generally good, although both Styre's deflating head and the decidedly plastic contraption Harry steals could have been made slightly better. None the less, the most frequently criticised mistake of this story- Sarah's apparent recognition of the rather different looking Styre as her old... acquaintance Linx is actually rather less of an error when you notice that her initial cry is made when he is still wearing the helmet- at which point they are identical. As to why she maintains the belief later- well, if you had only met one Sontaran, didn't know they were cloned, and had just been captured by a giant steel robot and dragged across the countryside, is it not a mistake that you too might easily make?
It's a slight story, but well repays the time spent watching it.