Reed has previously written stories (included in The Dragons of Springplace, 1999) about a galaxy-cruising, supercolossal spaceship: this is the novel. Of unknown origin, the ship is nearly as ancient as the universe itself; when it wandered near the Milky Way, various human factions fought to take possession. The dominant group evolved into the ship's Captains and crew—able to regenerate themselves from a few scraps of brain tissue, they're all but immortal—who then took aboard human and alien passengers on millennia-long cruises through the galaxy. Now, the Master Captain orders her senior captains to assemble in secret for a confidential mission: to explore the astounding planet, Marrow, discovered to be at the heart of the ship itself. Mars-sized, composed almost entirely of molten iron, barely habitable, Marrow is protected by energy fields that unexpectedly destroy the explorers' equipment and means of egress. No rescue attempt materializes. But, thanks to Marrow's weird cycle of development, in five thousand years the castaways will have an opportunity to escape. Meanwhile, they'll be forced to reinvent civilization, science and technology, and endure rebellious children and their stories about the ship's Builders and their mortal enemies, the Bleak. Only then can they begin to unravel why they weren't rescued, and discover the truth about the ship and its Builders.
The plot may not entirely hang together, but this is a wonderful adventure, with idea piled upon splendid idea in a continually fascinating narrative.