A classic of alternate worlds. . . we open with the head of the Paratime Patrol contemplating his job, which he is about to hand on. And then we have the Princess Rylla in the war council where her father and his councillors face a hopeless war. And then we meet Calvin Morrisson, Pennsylvanian cop, going in with three others to take a murderer. Except that he's accidentally swept up by a Paratime traveler, and dumped in another world.
Recognizes it as Pennsylvania. But then he meets people and realizes they are such as never lived there. And gets caught up in fighting. And gets shot.
By, it turns out, Rylla, who is properly ashamed of herself. He learns that they refused to surrender sulphur springs to the priests of Styphon, and will be crushed. The priests have given their enemies fireseed, and refused any to their country. Fortunately for them, Calvin -- now going by Kalvan -- knows how to make it because it's gunpowder. And fortunately for him, he was a big military history fan.
Meanwhile, there's a parallel plot about the Paratime operative dealing with his case.
It involves noticing something about mines, how the prince needs a son, usury, the difficulty of keeping secrets, taking an old fortress, improving how cannons are used, considerations of rank, betrothals and more.