As much philosophy fiction as science fiction, the book is about the adventures of a crew of almost-humans from Earth, each with special powers, evolved or engineered, though not exactly super-heroes.

They have been sent to investigate probably hostile aliens.

As well as a struggle against beings so alien as to be almost impossible to understand, the book covers the nature of consciousness, the relationship between intelligence and consciousness, the implications of optical and similar illusions from other senses and more