As episodes go, I think this one is the most even. It is certainly my favorite so far. We get final confirmation of what has been happening all along. Alien ambassadors have been kidnapped by humans and the astronauts are being held prisoner until their return. The Doctor, finally having all the necessary pieces of the puzzle, can now work toward a resolution. This is good stuff.
Once more, the aliens are not necessarily evil. At this point, we don’t know if the aliens were hoping to make peaceful contact with Earth or not, but the fact that ambassadors are involved means some attempt at negotiation. The aliens haven’t responded with violence, merely taking hostages of their own. They have not, however, eliminated the possibility of violence, as they tell The Doctor that they will destroy the planet if their people are not returned.
While there is not an actual invasion happening, the threat of invasion is what is driving General Carrington. It seems he was behind the abduction of both the aliens and Liz. He fears the alien ship is an invasion force and is reacting accordingly. He is being driven by fear and what he feels is best for the country he is trying to protect. It just a shame he got things so wrong.
So, the story has really picked up—for me—in the end. I’m looking forward to the resolution.