More than 700 years after the 21st century, humanity has spread across the galaxy, first aboard "Hawking drive" ships and then, after Earth is destroyed in a scientific accident (the Big Mistake), through what that accident was studying: controlled singularities known as "farcasters", which permit nigh instantaneous travel between points separated by apparently unlimited distances. The farcaster network (the "WorldWeb") profoundly changed the Hegemony of Man (the political entity headquartered on the planet Tau Ceti Center which controls all worlds in the WorldWeb and many beyond), permitting such wonders as the river Tethys—a river which flows uninterruptedly through farcaster portals across dozens of planets. Also flowing across these portals are the strands of fabulously advanced computer technology which support the datasphere (a network reminiscent of the Internet in design, but far more advanced), in which lurks the powerful, knowledgeable, and utterly inscrutable —the vast agglomeration of millions of the most intelligent AIs who run the farcaster networks and nearly every other piece of high technology.
The Hegemony itself is a largely decadent society, relying on its military ("FORCE") to incorporate into the WorldWeb the colony planets, even unwillingly, and also to defend the Hegemony from attack by the Ousters, quasi-transhumanist "interstellar barbarians" who dwell free of and beyond the bounds of the Hegemony and shun all the works of the TechnoCore (especially farcasters). The political head of the Hegemony is an executive advised by the TechnoCore advisory council, and working with the "All Thing" (a real-time participatory democracy, much like a web forum, which is enabled by the TechnoCore's datasphere, and reminiscent of the medieval Icelandic Althing), and the senators that the All Thing elects. All is not well in the Hegemony, however. All the TechnoCore's advice and predictions are confounded by the mysterious structures and a creature on the remote colony world Hyperion (named after John Keats' poem) -- the Time Tombs and the Shrike (respectively) -- which were sent back from the future by an unknown party for unknown reasons. Even worse, the Ousters have long been obsessed with Hyperion, and their long-planned invasion is imminent.
Into this turbulent situation come those who will be the last seven pilgrims to make the journey to the Time Tombs and the Shrike, there to ask on
Aboard the treeship the pilgrims finally meet after being revived out of their cryogenic storage state; they decide they each will tell their tale to enliven the long trip to the Tombs and to get to know each other. By lots, the first to tell his story is the young priest of the dying Catholic faith, Father Lenar Hoyt.