Lest Darkness Fall is similar to Mark Twain's A Connecticut Yankee in King Arthur's Court . 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.
When Padway arrives, Italy is ruled by the Ostrogoths, a Germanic tribe which has recently overthrown the Western Roman Empire. The novel depicts their rule as a benevolent despotism, allowing freedom of religion and maintaining the urban Roman society they had conquered.
In the real timeline the Byzantine or Eastern Roman Empire temporarily expanded westwards, embarking on what came to be known as the Gothic War (535–554). They overthrew the Ostrogoths and the Vandals in north Africa, but this war devastated the Italian urbanized society that was supported by a settled hinterland, and by the end of the conflict Italy was considerably depopulated: its population is estimated to have decreased from 7 million to2.5 million people. The great cities of Rome were abandoned, the Byzantines never fully consolidated their rule over Italy, which faced further invasions by the Lombards, and Italy fell into a long period of decline. Some historians consider this the true beginning of the Dark Ages, at least in regards to Italy.
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.
Padway rescues the recently deposed Thiudahad and becomes his quaestor. He uses the king's support to gather forces to defeat the formidable Imperial general Belisarius. Padway managed to surprise Belisarius with tactics never used in the ancient world. Then, deceiving the Dalmatian army, Padway reinstates the senile Thiudahad and imprisons King Wittigis as a hostage. In 537, when Wittigis is killed and Thiudahad descends into madness, Padway has a protégé of his (Urias) married off to Mathaswentha and then created king of the Ostrogoths. He also tricks Justinian I into releasing Belisarius from his oath of allegiance and quickly enlists the military genius to command an army against the Franks.
The landing of an Imperial army at Vibo led by Bloody John and a rebellion led by the son of Thiudahad threaten the Ostrogothic kingdom and its army is destroyed at Crathis Valley. Padway assembles a new force, spreads an "emancipation proclamation" to the Italian serfs, and recalls Belisarius. The armies clash near Calatia and then Benevento. Despite the lack of discipline of his Gothic forces, some simple tactical tricks and the nick-of-time arrival of Belisarius secure Padway's victory.
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.
Europe will not experience the Dark Ages due to Padway's actions; darkness will not fall.