A pretty enjoyable start, tiresome and somewhat tedious middle and a very nice and heart-warming ending. I'm not sure what to make of this one. There's stuff I liked and there's stuff that I didn't!
I enjoyed seeing the Doctor get into all those fights! The first one with the primitives was especially exciting, and unexpected too!
The captain was wonderfully cast; he was stunningly convincing at being cold-hearted and selfish. I also really enjoyed Caldwell's character and I'm glad he had a happy ending!
Again, I really enjoyed the Master in this one, even if we have no idea why he's there until the last episode. He has some great scenes with the Doctor, as usual.
My favourite scene, however, was seeing the colonists had actually survived and were not on the ship when it blew up after all; when it blew up, I actually felt quite sad, so seeing them all survive and win in the end was a delight.
There are cons to this serial though. It certainly suffers from being a couple of episodes too long. The mid section of the story is very slow. I think that was probably because there were big overall questions that weren't answered until the last half of the last episode, such as what does the Master want & why is he there, why isn't the planet producing any food, etc - considering this is a six part story, not knowing key bits of information about the story throughout the majority of it made it slow and a bit hard to get through. It's worth it in the end, but it's still a major flaw.
Another problem I had was with the primitives... I feel like had they not been there, the story would hardly have changed. They were unnecessary and irrelevant most of the time, only there to basically give a reason for the doomsday weapon, making the whole 'human sacrifice' thing very unnecessary and boring.
On the whole, it's a good story. I enjoyed it. Although it can be slow at times, it's worth it for the denouement. I would certainly watch this story again, but there are certainly others I'd rather watch instead.
3/5 from me.
