My enjoyment of the series starting dipping a bit starting with this book because May took great pains to establish certain rules in the first volume and then started playing a bit loose with them here.
One of the rules was that no human who had metapsychic powers could ever use the one way time gate. The reasons for this are not really well established, nor is it ever explained why none of any of the alien races ever wanted to go into the past (with the aliens you could argue that the Mental Unity the other races had, which was only an imperfect state for humanity, precluded them from seeking to use the time door, but no one, out of billions of entities?), but OK we can just roll with it. However, one member of Group Green, Elizabeth Orme, from the first book was a Grand Master Farsensor (really powerful astral projection and being able to find people) who was nearly killed in an accident, suffered massive brain trauma, and lost all her powers. She was allowed to use the time gate and the temporal shock returned her mental abilities.
At the time when the series begins, about 2% of humanity has mental powers. Masters have an ability (farsensing, telekinesis, coercion, redaction/healing, creativity) that is very powerful. Grand masters are those whose powers are much greater than Masters (think Olympic level athletes) and paramount grand masters are the ne plus ultra, basically the top 2 or 3 out of billions upon billions.
In this Group Green there is Elizabeth, already an anomaly, but also two others Felice Landry and Aiken Drum. Both of these characters become much more important as the story progresses and they both possess at least grand master if not paramount grand master abilities in multiple powers. It felt a bit too deus ex machina to have all of them in the same group, not to mention that I find it a little incredulous that a society such as the one May introduced would allow individuals, even latent ones, as powerful as this to enter the time gate.
One aspect I did like was how this volume ended, with Felice wreaking vengeance on the Tanu race and using her powers to open the straits of Gibraltar to create the Flood that is part of so many myths from around the world. Another is the revelation (which had already been heavily implied) that humanity’s mental powers come from the Tanu & Firvulang. It is their contribution to humanity’s genetic pool, possibly up until the coming of homo sapiens, that will eventually allow them to access psychic powers and join other races in the stars.
While it is necessary to advance the story, this felt like the weakest volume of the series