- You may be looking for the unproduced audio story.
According to Astrolabus's life story as he told it to the Sixth Doctor, which contradicted what the Doctor knew of Astrolabus's past as a Time Lord, the city of Alexandria became a crossroads in Time due to the effects of a planetary conjunction. This allowed the "Visitors" to descend upon the city in their skyships. (COMIC: Voyager [+]Loading...["Voyager (comic story)"])