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. Padway initially wonders if he is dreaming or delusional, but he quickly accepts his fate and sets out to survive. His first idea is to make a copper still and sell brandy for a living. He convinces a banker, Thomasus the Syrian, to lend him seed money to start his endeavor. He teaches his clerks Arabic numerals and double entry bookkeeping.
Padway eventually develops a printing press, issues newspapers, and builds a crude semaphore telegraph system utilizing small telescopes. However, his attempts to reproduce mechanical clocks, gunpowder, and cannons are failures. He becomes increasingly involved in the politics of the state as Italy is invaded by the Imperials and also threatened from the south and east. At the end of the novel Padway has stabilized the Italo-Gothic kingdom, introduced a constitution, arranged the end of serfdom, liberated the Burgunds, is having boats built for an Atlantic expedition (Padway wants tobacco), and the king of the Visigoths has appointed Urias as his heir.