A very complicated and compelling plot with a very high-concept alien species; I found the revelation of it all deeply satisfying and was left with a strange sense of longing at the end. The setup felt like a bit more of a slog, unfortunately; there was a lot of back-and-forthing. I was also a little put off by the feeling that all of the named characters were meant to be references to the Doctor's old friends, but not enough to identify any aside from Christina and the Brigadier.