From the Depths of Time, Part One

Shortly after an exploration ship visits the planet Copernicus a xenoarchaeological expedition on Turilsus is attacked by an alien menace, starting a chain of events that threatens the human race in the past, present and future.

When a visit to the distant future reveals that the Centaur race has returned from a millennia-long absence, and are hunting down the last remnants of humanity, the crew of the time ship Valentinian consider themselves obligated to apprise Temporal Security in the twenty sixth century of the impending threat before it arrives, but when Earth is attacked it seems it's already too late. With the means of invading time at different points simultaneously, the Centaurs are able to stage multiple invasions across human history, making themselves unstoppable.