A working title, used on some advance covers for the British edition, was The Tides of Tiber.

Evidence is found of an intelligent alien species who visited the Earth long ago and left an encyclopedia with the collected knowledge of their culture.

9000 years ago the alien society in the Alpha Centauri system was under threat of cosmic bombardment.

As all crew members were required to bring several projects to work on, due to the decades long nature of the mission, she spends her time writing biographies of several family members who were closely involved in the acquisition of a repository of the aliens' knowledge.

As another project, she translates two autobiographies by the aliens who had visited the Solar System over 9000 years earlier.