In this novel we have a protagonist who has a massive dose of survivor guilt (something that seems to be a common theme in this series): born and raised on Mars, he stormed out the settlement's habitat after an adolescent argument with his family only to watch helplessly as the settlement was crushed beneath a landslide. Now he's trying to redeem himself by participating in a mission to save the inhabitants of a distant world from the Death Wave that is rushing out from the galactic core.
The system is very odd, with a gas giant near the star and two Earthlike planets in highly eccentric orbits that almost cross at one point. One planet, named Gamma, is inhabited by people living in scattered farming villages, barely above the Paleolithic.
Our protagonist discovers a species of octopus-like sophonts living in the liquid layers of the gas giant, and argues that they too deserve protection against the Death Wave. Then he goes to Gamma and ends up not only contacting, but also befriending the inhabitants of one village. He learns of their peculiar life cycle, in which each generation dies when monsters come from the other eccentric planet, Beta, during the storms that accompany their new approach. Only one person, the Rememberer, survives the long winter to teach the new generation, which hatches from the ground the following spring, as Gamma once again approaches its primary (with the extreme eccentricity of its orbit, distance from the primary is more important than axial tilt in determining the seasons).
When he presses the issue, he is told that the Gammans once built great villages all over the world, with very tall buildings. Then the Sky Masters came and crushed them, and ordained this way of life for them. At first glance this would seem to be mere superstition, a supernatural explanation for the brutal realities of their system's astrophysics and its effects upon their lives. But as the novel progresses and the monsters turn out to be quite real, it becomes clear that these creatures were the handiwork of a very sophisticated technological species. Furthermore, there is no star or micro black hole anywhere in the right position to have provided a natural explanation for the disrupted dynamics of the system. The only possible explanation is a long-ago interstellar war.
The novel ends on a hopeful note, but I for one would really like to know more about the Sky Masters. A species that powerful, with technology beyond even what the Predecessors have provided humanity, probably would've spread beyond a few systems, so there should have been evidence of them on other worlds in the stellar neighborhood, even if they have been destroyed or have self-destructed. Unfortunately, now that Mr. Bova is deceased, it seems unlikely that we will ever learn more of this mysterious people, of how far they fared and whether they had an effect on any other species' development, let alone whether they are still out there somewhere.