Stories about Roman Britain for young readers are often set in the early years of Roman occupation, and sometimes use a protagonist from the present, either by time travel or by what Hellekson terms the ‘parallel worlds story’. These narratives need to navigate the nature of Roman influence on Britain, showing it as either brutalising or civilising. John Christopher’s Fireball (1981) focuses not on the early years of Roman occupation, but on an unspecified later period, when Roman rule was the accepted norm. Fireball portrays a contemporary parallel world, but one which feels like the distant past. The text thus combines the time travel narrative with the parallel worlds narrative, and paints a rather ambiguous picture of the Roman occupation of Britain. I examine how Fireball navigates the transition between time travel and parallel worlds, and how this generic shift determines the way in which Christopher portrays the Romans.