Warning: Wikipedia contains spoilers
The novel begins when the SETI program picks up broadcasts of music. The first expedition to the world that is sending the music is organized by the Jesuit order.
The expedition meets the aliens. Only one of the crew, a priest, survives to be brought back to Earth, and he is damaged physically and psychologically. The story skips back and forth between 2019 when the signal is first discovered and 2060 when a Vatican inquest is begun in order to find the truth.
The novel gradually reveals the nature of the alien society, and what happened to the expedition.
Children of God is the sequel to this book.