The West Virginia town of Grantville, torn from the twentieth century and hurled back into seventeenth century Europe, has allied with Gustavus Adolphus, King of Sweden, in the United States of Europe. So, when Gustavus invades Poland, managing to unite all the squabbling Polish factions into repelling the common enemy, the time-lost Americans have to worry about getting dragged into the fight along with the Swedish forces.