In a future world being evacuated by spaceships, four travelers try over and over again to
get to the evac point, each time with all of them being slightly older versions of
themselves.
What this mean, Howard explained, was that the traveler could only jump to a time and
place where they had previously existed. The traveling version of the person would take
the place in the world of the old version, with all the knowledge they had gained since
that time kept intact.