Imagine a parallel timeline where there exists a Roman empire that never fell, a Hitlerís Germany that was never defeated, and an America where President Kennedy was never assassinated. That last one I just threw in because it's one I would like to visit if I could. But I digress. Gunpowder Empire follows Jeremy Solters, a teenager in the late twenty-first century. He and his family live in California during the school year but spend the summer on the frontier of the Roman Empire.
This process of time travel is called Crosstime Traffic, and families are trained in groups to pass as locals so as not to be detected. The Solters' main problem is that they are not willing to own slaves like the rest of the Roman citizens, but all in all they get along all right - until Jeremyís mom gets sick, and antibiotics will not cure her. So both parents go through the gateway to get her to the doctor, but the gateway breaks and they cannot get back. Then they cannot communicate from home to Rome, which leaves Jeremy and his sister all alone in a besieged city.
Gunpowder Empire has an original plot that alternate history master Harry Turtledove develops convincingly with historical detail and believable dialogue. The settings are so well described the reader can almost see the togas flowing around the senators as they stand on the senate steps. This is an excellent novel.