Anderson's latest is the first novel set in his popular ""Time Patrol"" series, based on the premise that time travel makes necessary a sort of police force to prevent dissident groups from attempting to after current reality by changing crucial past events. Manse Everard--an Unattached Agent of the Time Patrol--travels through history to crisis spots, preserving the future by stopping those who would meddle with the past. Early in the novel, he recruits a new agent, Wanda Tamberly, who plans to catalog the natural history of past eras for the use of time travelers; it soon becomes clear, however, that her role in the Patrol's mission is to be more complex. The paradoxes of time travel make the plot all but impossible to summarize briefly, as the action jumps from the near past (California, 1987; Afghanistan, 1085) to ancient history (Asia Minor, 209 B.C.) to deep prehistory (13,210 B.C. and even earlier), with several alternate time lines thrown in for good measure. The conclusion is satisfactorily complex--and does justice to the more somber implications of the Patrol's mission. Anderson is a master of the ingenious plot twists implicit in time-travel stories, and a consistently readable (if sometimes long-winded) novelist. A solid piece of work from an old pro.