From the Depths of Time - Part One — Marius Smith

Curious about what the future holds, the crew of the time ship Valentinian travel to the year 2791 and discover that the galaxy has been invaded by the ancient Centaur race whose existence they learned of on the planet Copernicus in a previous timeline. Unable to prevent the invasion with just one ship, they resort to going to the year 2568 to contact Temporal Security to enlist their help. When the Centaurs reach Earth with such an overwhelming force that the outcome seems inevitable, Valentinian's crew set out to investigate the Centaurs' interest in the planet Turilsus, home of the lost Karathi civilisation, and the planet Amazonia, site of an early manned space mission from Earth in 2142. After learning that the Centaurs had begun setting the scene for their invasion of Earth some time before their arrival, William and his crew go back in time to investigate the matter further and search for a way to undermine the invasion before it takes place.

The Centaurs' mastery of temporal physics allows them to invade multiple time zones simultaneously. With the rules of causality being rewritten, nowhere in history is safe from the Centaur menace, as the crew of Valentinian quickly find out.