Well now! Didn't see that coming! Or that! Not going to tell you what (don't want to spoil the surprises), but this is a great story for those `wait a minute!' moments.
It's quite hard, I'm finding, to review this story without giving away spoilers, so this may sound a bit cryptic, but bear with me.
The mad people in this story sound reeeealllly insane! And that's a good thing in this story, believe me.
The bad guys sound like you wouldn't want to meet them in a dark alley; or even an alley fully lit, to be honest.
And the Doctor, and Ace (and some other people who pop up who have met the Doctor and Ace before - but don't worry if you haven't) are in fine form.
This is a great story - alien planet, strange goings on, new bad guys, familiar bad guys, alien presences that have been waiting for their moment for revenge, evil plots so convoluted you would think they'd have to write them down to remember them, weird people, lots of gory deaths, and at the end - well, the Doctor and Ace trundle off in the Tardis again for another adventure. But that's the way it's always supposed to be in Doctor Who, isn't it?
Great story; great characters; great writing; great sound effects - wonderful listening. Can't go wrong.