Review of both episodes:
It was an experiment to have the first two-part story for many years and to film the entire story on location in an outdoor setting (Dartmoor National Park). These experiments work well as the location work is very good and the two part story is very neat with no waste material or time for boredom. This never quite manages to reach 10/10 classic status because it is quite unremarkable as a story but it is very interesting, thoroughly entertaining and very well done so I rate it very highly.
The story follows on from The Ark in Space. The Doctor, Sarah and Harry travel via Transmat to Earth from space station Nerva 10,000 years in the future. There they find a tiny group of humans some of whom have been captured and tortured by a mysterious alien that turns out to be a Sontaran. The experiments are being done on the humans to find their weaknesses in preparation for a Sontaran attack on humanity.
Some criticisms I have read are not deserved in my view as for instance I have seen it said elsewhere that it does not make sense the Sontarans are wasting time testing humans when there are very few surviving on Earth but it is clearly said in The Ark in Space that the humans on Nerva are the chosen people to take humanity forward but not the only humans left. It is clearly said in that previous story and in this one that there are other human colonies and it is also explained that the Sontarans plan to wage war on the humans elsewhere not just those living on Earth. So this all makes more sense than some critics claim. It does seem a slightly inefficient and unnecessary strategy but it is credible enough.
Another criticism is that this is dull and merely a filler. I do not find it dull. This is an entertaining story with a welcome return for Sontarans and very good performances. It is gritty and bleak looking but I rather like that and it fits the story that Earth has been abandoned. I find the story fun. It is a filler but a really good one because it is short and acts as an extension of The Ark in Space which also links in to the later story Revenge of the Cybermen.
It is not a 10/10 classic but is very good and neatly presented as a tight two parter. The acting is excellent. Tom Baker, Elisabeth Sladen and Ian Marter are all wonderful in the regular lead roles and this adventure strengthens the relationships developing between their characters. Sontaran Styre is brilliant visually and in the performance of Kevin Lindsay. He acts the part perfectly delivering dialogue really well and enacting fight scenes and torturous experiments well. The Sontaran face moves perfectly with speech and the overall costume is great for its day. The human colonists are well believable and engaging and the idea is interesting. Grim torture, the threat of war, the return of an excellent alien race and involving characters make this really good in my opinion.
Both episodes 9/10.