American archaeologist Martin Padway is visiting the Pantheon in Rome in 1938. A thunderstorm arrives, lightning cracks, and he finds himself transported to Rome in the year 535 AD. At this time, the Italian Peninsula is under the rule of the Kingdom of the Ostrogoths. The novel depicts their rule as a relatively benevolent despotism, allowing freedom of religion and maintaining the urban Roman society they had conquered, though slavery is common and torture is the normal method of interrogation by what passes for law-enforcement agencies.