At the end of CV, the single parasite had managed to reproduce via the unborn child of one of the passengers, before it was trapped at the bottom of the ocean.

It begins to learn about the human world by sampling the minds of people from all walks of life (hence "The Observers"), changing them in the process.

The scientists in charge of learning about the "disease" are well-intentioned, but Knight shows their darker side as well, as they gradually move from benign observation to more coercive techniques, including torture.

The parasites decide collectively that in order to save the human race from its own bad impulses, they must put a stop to violence.

The Encyclopedia of Science Fiction [1] writes that "the series as a whole seems young at heart, and Knight's cognitive energy remains clearly evident".