Tractites; a peace-loving quadrupedal herbivore race who mean no harm to anyone. But unfortunately, they appear to be living on
a soggy islandin the middle of a planet that just ought to be Earth.
Back in datum-reality, a somewhat older Jo Grant is using her UNIT connections to try and track down a couple of archæologist friends who sent her a worrying message from the field.
This is a fairly simple time travel plot with the twist that the usurping aliens are portrayed in a very favourable light, and one of the humans we are introduced to is probably psychotic. The time-travel plot at the core of it isn't really the story though; it's an exploration of ethics – should the Doctor really try to return the time-stream to the way it
shouldbe, with the Humans back on earth. Unfortunately it fails rather because we know that he has to – and in fact about half-way through it is obvious that if he doesn't then the entire Universe is endangered.
Sam gets another ethical work-out in the process, but we don't really learn anything new about her. But on the plus side Jo is portrayed as having grown since she parted ways with the Doctor, or at least changed. The Doctor doesn't get a very good characterisation either; whilst everything he does is Doctorish it doesn't really have the McGann Doctor's characteristic flamboyance or naïvity.
Not one of the best books, but there's nothing in it to dislike really either.
Oh, and like the previous three, there's a smidgen of a hint of future events, one that I never noticed the first time through….