I disagree with the other reviewers. While this isn't the very best of the 3rd doctor stories (i.e., not as good as The Silurians or Inferno), I think it's the strongest and most interesting of its season. It's certainly far better than the other 6-parter, The Mind of Evil, which is plodding and repetitive at times. Here, we have a story that finally justifies its length due to the number of elements involved - the colonists' effort to survive on a barren planet, the mining corporation's attempt to destroy the colonists to make way for their own enterprise, the ancient buried alien civilization fallen into primitive decay, and the Master's effort to find the Doomsday device. And the aliens don't look too bad - especially the little shrunken one at the center of the underground city.