An expedition sets out to discover what has happened to a lost colony on a distant planet. Amongst the scientists is a strange, intense teenager called Mark Annuncio, raised to be a kind of human computer, able to make connections between different specialisms. Disliked by the crew, Mark nevertheless may be the only person capable of saving the ship when they land and scientists begin dying mysteriously.

Sarah and David Kirsten are tourists from the 21st century who take a forbidden holiday in 1938 Mexico. But enforcers from the future, trying to keep the time-stream free from pollution and paradox, are on their trail. Can they evade their pursuers without betraying their origins?