Weeks after their meeting with Captain Cook, the First Doctor, Ian, Barbara and Susan land in the city of Alexandria during the 5th century, at a time when the famous library is still standing. Deciding that they would all like to stay and enjoy the city for a time, Ian obtains work in customs while the other three enjoy the lost knowledge of the library and the local arts and theater.
And then a book is discovered, one which should not be present at that time and place. A book not written by humans, and as the Doctor gets ready to leave and take it with him, the owner arrive and it becomes very clear just how the ancient library was destroyed...
- Historical-Domain Character: Hypatia, a real-life historical figure, notable for being a very rare woman teacher of mathematics in her century.
- National Stereotypes: Hypatia thought all Britons wore no clothing and blue paint. Ian assures her that's only when they go to war.
- Not a Date: Ian keeps insisting that he wasn't on a date with Hypatia, they were just talking. The Doctor and Barbara aren't buying it.
- Screw the Rules, I Have Connections!: The attitude of the young Roman whose books Ian impounds due to the Roman's failure to obey the law in Alexandria.
- Shapeshifting: The Mim do this, intially appearing from the sea in squid-like forms, and pass into legend as sea monsters.
- The World Is Not Ready: Played straight. The knowledge in the Mim book is too advanced for humanity of the 5th century. The Doctor is prepared to take the book to prevent the knowledge getting out, while the Mim take the more drastic measure of attacking Alexandria and burning down the library to protect their planetary survey information.