Warning: Wikipedia contains spoilers
The plot revolves around 7 characters (Father Lenar Hoyt, Colonel Fedman Kassad, Detective Brawne Lamia, Poet Martin Silenus, Scholar Sol Weintraub, a Consul, and Templar Het Masteen) all of whom are on a pilgrimage to the planet Hyperion to make a request of the Shrike. The Shrike is a being made of metal who has the ability to manipulate time, and is notorious for murdering people in gruesome ways.
By itself, Hyperion is an incomplete novel. The editors insisted that the original story was too long and had it divided in two, half-way through the pilgrimage. Since the second part wasn't long enough to constitute a novel by itself, Dan Simmons expanded it with a parallel storyline involving the character Joseph Severn as well as several stretches of exposition and recapitulation. As a result, many readers believe this sequel, published as The Fall of Hyperion, to be much weaker than the first book, if not the weakest of the entire series.