Millennium features a civilization that has dubbed itself "The Last Age". Due to millennia of warfare of every type (nineteen nuclear wars alone), the Earth has been heavily polluted and humanity's gene pool irreparably damaged. They have thus embarked on a desperate plan; time travel into the past, collect healthy humans, and send them to an uncontaminated planet to rebuild civilization.
The time travelers can only take people that will have no further effect on the timeline: those who have vanished without a trace, or died without being observed; otherwise they would be changing the past, which risks a temporal paradox and perhaps even a catastrophic breakdown of the fabric of time. Though they collect everyone they can, they exert a great deal of effort on those destined to die in various disasters such as sinking ships and crashing airplanes (and once a century of Roman soldiers lost and dying in the North African desert). As such incidents leave no survivors to report interference and change the timeline, they can freely remove the living but soon-to-die victims, and replace them with convincing corpses they have manufactured in the future.
The novel deals with several of the raids, their inevitable discovery in the present day, and the fallout that results from changes to the present day reverberating into the future.