A fine story, perfectly enjoyable but nothing special.

I think the thing I enjoyed the most about this serial was how often it made me think of a much better Star Trek episode that pulled off the concept of fighting an endless war against a secret machine much better in a third of the time.

I've been waiting for a sentient segment of the key since the very beginning, and I was a little disappointed by how little they seemed to do with the concept. She sacrifices herself willingly and the Doctor and Romana are barely forced to grapple with the implications before everything resets.

The shrinking was clearly added to pad the runtime but I enjoyed Drax and learning a bit more about the Doctor's history.