Under the thick ice crust of the moon Ilmatar, there is a world-spanning cold ocean filled with strange alien creatures, some of them intelligent with primitive technology. A human expeditionary base studies this dark and submerged world under strict non-contact rules. Inevitably, one of the academics tries to get a bit too close. At the same time, a ship from the advanced Sholen race arrives at Ilmatar. The Sholen with to contain human contamination of pristine worlds, and due to internal Sholen politics they attempt to force the humans to leave.
The sub-ice ocean of Ilmatar is richly described through the diverse viewpoints of Ilmatarans, humans and Sholen. Mr. Cambias has managed to make the aliens quite alien in their thinking, especially the Ilmatarans with their immediate sense of experience, tricky relationship with memory and sudden narcolepsy. An adventure novel at heart, this book manages to ride above mere action into more intellectual territory by exploring the relationships between three races with different motivations in an unusual environment. It is also a good first contact novel. I was strongly reminded of the film The Abyss, with its mix of dark underwater milieus and dank submerged base.