"In less than 25 years the Romans have invented electricity generation, airships, radio and who knows what else. Is that reasonable?"

Ancient Egypt, 41 BC. The Doctor and Peri watch as Cleopatra's pleasure barge glides up the Nile in preparation for her fateful meeting with Mark Antony. And an alien presence observes the TARDIS, waits for it to dematerialise, then pounces.

When the time ship lands, the Doctor and Peri find themselves in ancient Rome, in the tomb of Cleopatra. But something is very wrong. The tomb walls depict steam-driven galleys and other disturbing anachronisms. The Roman Empire is preparing for a devastating war — using weapons from the future capable of destroying the entire world.