Sextus discovered a man named Thrasyllus, who was a natural savant, and taught him navigation using the stars. He planned to use Thrasyllus' calculations to help himself get back home. He decided to change Earth's history, allowing it to develop faster.
When Sextus found Tegan and Turlough, who had been separated from the Fifth Doctor, he captured them in order to get to the Doctor. Sextus's plans were defeated by the Doctor with the help of Felix and an orphan boy. (PROSE: Rome)