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.

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.

Upon returning to her present (our future), Louise is informed that the weapon that was left behind has caused a paradox and that it must be recovered to prevent a breakdown in the fabric of time.

The theme has been so extensively explored, in fact, that I found it no trouble to write a book with chapter titles borrowed almost exclusively from the long list of stories that served, in one way or another, as ancestors to this one."

John Varley worked on numerous drafts of a screenplay for over ten years and half a dozen directors until it was finally filmed by Michael Anderson in 1989.