When Tom Ryan stops his car late at night on a dark road for a man dressed as a Roman centurion, his first thought is that he's picked up one of those amateur re-enactors but the man, Marcus Appius Silvanus appears to speak only Latin. He insists the year is AD60 and that the British Queen is Boudicca - and that he and his men of the Fourteenth Gemina are in hot pursuit of her.
Tom and his sister Mary shelter the Roman, but inadvertently attract the attention of an unscrupulous journalist. He's not the only one interested in the Ryans: an IRA terrorist who was once Mary's lover in Northern Ireland tracks her down to tell her the plane crash which killed her parents twenty years ago was caused by the British security services.
Deep in the English countryside, those same servants of the state are busy exploiting the theories of a young prodigy to build 'Oracle', a probe that can view the past - and, they hope, the future, so that threats to national security can be stifled before they occur.