It postulates a road that travels through time, with a nexus placed every few years where a handful of specially gifted people are able to get on and off. While there is a plot involving a series of assassination attempts on the protagonist, the novel's main strengths are the unique nature of the setting, character development, and the short vignettes on each of the would-be assassins.
The book has withstood the test of time reasonably well because there are few references to technology. There are two semi-sentient electric devices that appear, that are implemented as physical books (with printed pages) that may be something of a stretch for a 21st century reader.