Lest Darkness Fall is similar in concept to Mark Twain's A Connecticut Yankee in King Arthur's Court, but the treatment is very different.

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 the normal method of interrogation.

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).

In addition to this specialized and uniquely useful knowledge of the current war, Padway has taken a general interest in military history, which he is eventually able to put to very practical purposes.

He eventually develops a printing press, issues a newspaper, and builds a crude semaphore telegraph system utilizing small telescopes.

In 537, when Wittigis is killed and Thiudahad descends into madness, Padway has a protégé of his, Urias, married to Mathaswentha and crowned king of the Ostrogoths.

He tricks Justinian I into releasing Belisarius from his oath of allegiance and quickly enlists the military genius to command an army against the Franks.

Padway assembles a new force, spreads an "emancipation proclamation" to the Italian serfs and recalls Belisarius after his defeat of the Franks.

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.

[3] The book has also been collected with David Drake's novella "To Bring the Light" in Lest Darkness Fall and To Bring the Light (Baen Books, 1996), with other works by de Camp in Years in the Making: the Time-Travel Stories of L. Sprague de Camp (NESFA Press, 2005),[3] and with works by other authors in Lest Darkness Fall and Related Stories (Phoenix Pick, 2011).

An E-book edition was published by Gollancz's SF Gateway imprint on September 29, 2011, as part of a general release of de Camp's works in electronic form.

Like "Apotheosis," it also involves a future time-traveler investigating Padway's temporal disruptions, only she thinks he's a criminal from her own time who set out to change the timeline on purpose - a crime punishable by death.

Direct responses to Lest Darkness Fall include the stories "The Deadly Mission of Phineas Snodgrass" (1962) by Frederik Pohl,[6] and "The Man Who Came Early" (1956) by Poul Anderson.

It ends with the fantastically overpopulated alternate timeline sending someone back to assassinate the title character, allowing darkness to fall for thankful billions.

The Anderson piece is a tale of an American airman sent by a storm (like Padway) to Saga Age Iceland; in this instance, the outcome proves tragic due to cultural misunderstandings and a poor fit between the protagonist's attempted innovations and local needs.

"[8] Stating that it "is an excellent introduction to Rome at the time of the Gothic invasion", Carl Sagan in 1978 listed Lest Darkness Fall as an example of how science fiction "can convey bits and pieces, hints and phrases, of knowledge unknown or inaccessible to the reader".