 The novel concerns a military expedition to the planet World, where aliens live who have developed a strange form of telepathy or collective unconscious, "shared reality", which causes piercing "head-pain" whenever "Worlders" attempt to hold strongly differing opinions.However, the expedition concerns a crash-landed alien artifact in the planet's crust which has uncharted powers, and may be the key to humanity winning a war against the "Fallers", a genocidal alien race.The "Probability" trilogy takes place in a galaxy that has been colonized by humans.