Three kids are training for a marathon. They run through a fog back in time to Ancient Rome (as one does). There they are, for no good reason, assumed to be slaves, and are pressed into the gang of gladiators out for a training run (who knew gladiators were taken for runs?). They meet Spartacus, who takes them under his wing. He belives they are from the future. One of the kids, a girl named Francie, has to fight in the arena. This is the catalyst that sparks Spartacus' rebellion. The gladiators fight their way out of Rome, and a fog comes up that lets the kids go back to their own time.
Here's the main problem with the book--there are three kids and two would be plenty. We start with Nina's point of view, and, not unnaturally, I assumed she would be the main character. Mostly, though, things focus on Francie, who likes pizza more than Nina does and who is not as naturally athletic or spunky. Nina seemed to have more potential....but it never happened. Bill did not need to be in the story at all. He was a waste of page space. In short, there are no interesting, well-developed characters.
Here's the second, slightly less main, problem--the emotional intensity of Spartacus' desperate position was growing....what would happen? and then-fog happened and the kids went back to their own time. Kind of a let down.
Here's the third problem--good time travel plays with the tension of being a stranger in a strange land. Apart from some questioning along the lines of "gladiators?" there is none of this tension here. It's the sort of time travel where all the language/clothing difficulties are magically smoothed over. Good time travel also offers at least some sort of reason for the thing to have happened, even if it is just a slight connection of sympathy or similarity. Here the only point of connection was long distance running, which seemed too slight.
The illustrations, by W.T. Mars, do not further enhance the story. As is clear from the cover, he seems to have trouble with arms.
However, if you want your children to learn about Spartacus they could read this. I learn history best through fiction, and feel slightly more Spartacus-literate than I did before. I had not, for instance, known that his wife was involved. (I've never seen the movie).