The plot revolves around a
Union Army regiment and an artillery battery from the
American Civil War, which got transported to an alien world. The 35th Maine
Infantry Regiment and the 44th New York Light
Artillery Battery travelled to a different world using a ship that emerged from a mysterious electrical storm. The alien world was populated by descendants of
medieval Russians (or "Rus"), who still lived at a medieval level of technology. They learned that there were various civilizations on this world made up of descendants of people from various eras of Earth's history. The Union soldiers eventually discovered that the Russians had been hiding the existence of the Tugar from them. The Tugars were ten-foot-tall aliens with a culture and technology similar to that of the
Mongols. They rode a never-ending circuit around the planet and subjugated the human population in their territory for food. The Tugars visited each human society once a generation and culled part of the population for food so that the humans stay docile, compliant and never advanced enough to challenge them. The Union men were horrified by this revelation and kill the Tugars advanced scouts. They supported a peasant rebellion against the Tugar-appointed lords and begin modernizing Russian society. When the Tugars arrived, they encountered a semi-modern army equipped with
cannons and
smoothbore muskets. In a hard-fought battle, the humans managed to prevail, weakening the Tugars forever. The next several books take up the story a few years later. The Russians had begun to explore the possibilities of a union with the
Romans who lived next to their territory and were spared from the Tugar depredation by the Russian victory. Meanwhile, they faced a renewed threat from the Merki, members of the same alien race as the Tugars. The Merki make the same circumnavigation of the globe as the Tugars do but in a zone further south. Once the Merki learn of the Tugar's weakening, they seek to take advantage of the Russians, realizing the disastrous effects that would come from an industrialized enemy. They seek to maintain their racial dominance over the humans through the annihilation of those who have learned the secrets of gunpowder and industrialization. The Merki are aided by a group of Union sailors who fled the Tugar war on the ship which brought the Unions to this world. In exchange for their survival, the humans help the Merki develop firearms and other technology. The Merki attack is more successful than the Tugar one and forces the humans to abandon the Russian territory in a scorched earth campaign, but eventually, they win with their use of
trains. Several years later, the expanding human Republic faces a new threat in the form of the Bantag. The Bantag are a Horde from even further south. They are led by a member of their race who arrived from another world that has a mid 20th-century level of technology. This alien, a soldier in his own world, assumes a messianic role among the Bantag and modernizes their society to equal, and even surpass that of the humans. He is familiar with
atomic reactors as well as
centerfire rifles. The Bantag scavenge engines from decaying cities abandoned by their people millennia ago and use them to power airplanes. The reader learns that the people of the Bantag had once been a technologically advanced star-faring race, visiting many different planets and seeding teleportation devices across them. The Bantag homeworld collapsed in a nuclear war but the teleportation devices remained sporadically active and ended up transporting various humans to the planet Vallenia (the location of the novel), though Vallenia was not the homeworld of the Tugar, Merki, and Bantag Hordes. Using their advanced technology, the Bantag attacked the Union, introducing steampunk
tanks and 20th-century
mortars to the world of Vallenia. However, the humans developed their own flying machines (mainly
blimps and early
Zeppelins) and organized a counterattack. The final book in the series,
Down to the Sea, takes place a generation after the arrival of the Union soldiers. With the children of the original regiment members reaching adulthood, they face a new threat from across the southern sea, the Kazan Empire, who have an early 20th-century level of technology and who also have a selectively bred slave race of human assassins, warriors, and spies called the Shiv. The Shiv are controlled by a nihilistic religious Order that seeks to control the mysterious Portals of their ancestors. The book ends as if a tenth novel was planned but never written and/or published. ==Titles==