Into paleontologist Richard Leyster’s Smithsonian office walks Harry Griffin with a cooler and a job offer. The job's hedged about with peculiar and restrictive conditions. And in the cooler is a head: not a fossil, but that of a real stegosaur! Seems that mysterious beings from the far future, the Unchanging, are offering a limited form of time travel to humanity: namely, access to the Mesozoic. Those, like Griffin, who travel in time can and do meet themselves, but paradox—the deliberate repudiation of a known historical record—is forbidden. If a major paradox occurs, the Unchanging will retroactively withdraw their offer. Leyster accepts the job but, not surprisingly, encounters complications: the enigmatic person who runs the security and police side of the operation, the Old Man, is an older version of Griffin himself; a fellow paleontologist, Gertrude Salley, develops an insane hostility towards Leyster; Creationist terrorists, whose moles permeate the operation, smuggle a bomb into Leyster’s equipment, stranding him and his team in the Cretaceous. Salley meets an older version of herself, Gertrude, who seems to know what's really going on; with Griffin, they pledge to persuade the Unchanging to mount a rescue mission to retrieve Leyster. But to do that, they'll have to travel further into the future than anyone has yet penetrated—and deal with another whole slew of surprises.
Oddly structured and curiously undramatic. Still, Swanwick’s redevelopment of his Hugo Award winner, “Scherzo with Tyrannosaur,” bulges with intelligent speculation and intriguing plot twists: fans of the author, the original short story, dinosaur buffs, and time-travel aficionados will pounce.