Review of all 4 episodes:
It is very nice to get a return for the Sea Devils and the Silurians who had been in great 3rd Doctor stories. Here they meet the 5th Doctor as he arrives and is caught between the reptile races and the humans on a future Earth. The adventure starts very well but has issues along the way that are disappointing.
The best aspects are the ideas from writer Johnny Byre with returning 'monsters', a future Earth setting, themes of war and peace and some nice intervention work by the Doctor. Those ideas if presented really well and allowed to be shown to their full potential would give a really good adventure. Unfortunately, this story is not executed on screen as well as it could have been. Director Pennant Roberts manages to achieve some great scenes such as the excellent cliffhanger at the end of Part 1 but overall has too many clumsy scenes with some very dodgy action sequences which mainly occur in Part 3. The creature the Myrka, which would be fine on paper, is terrible. The budget and technology available is the main problem. It could have been done better with more skill, care and imagination but they were up against it. I do not judge this harshly in itself as it is not something that was easily remedied at the time and could be retrospectively improved with added effects. However, it does not help and the problems with the action scenes with the Myrka are poorly directed and have dodgy acting from guest actors which is something that really detracts from Part 3. Parts 2 and 4 also have some unimpressive scenes with actors doing poor falling over etc. It is a real shame because I actually think the story ideas could have made for a strong adventure.
As it is the first episode especially is still very good in my opinion and there are lots of good bits in the script/ideas. Peter Davison is on good form and I find a parts of the serial enjoyable.
My ratings: Part 1 - 8/10, Part 2 - 7/10, Part 3 - 4/10, Part 4 - 7/10. Overall - 6.5/10.