The port of Alexandria, 5th Century AD.
The Doctor, Ian, Susan and Barbara have taken a break from their travels, and are enjoying a few weeks in the sunshine – and the chance to appreciate the magnificent Library of Alexandria.
Ian also takes the chance to enjoy friendship with the philosopher Hypatia - but things here will not last forever.
The time travellers know that the library will soon be lost to history.
What they are about to discover is the terrifying reason why…