The Plague of Justinian in Constantinople

Ok, so not everything in the narration is accurate, but it is still not a bad introduction. They are exaggerating Justinian’s successes. His reconquest of the West was indeed a reconquest, not a liberation of occupied Roman territory. The empire was long gone in the West before Justinian and Constantinople is unlikely to have been able to hold even the Mediterranean  part of the  former empire without the plague.