Thomas and Marcie discover they are not in an ancient tomb but in some form of underground installation. Pendragon, Inga and their workers follow the children underground. Pendragon has been here before - she recalls when the place was alive with activity. She instructs Inga that the boy is important, but the girl is dispensable... Miss Pendragon finds what she has been looking for - the Behemoth. Far from being a monster of legends, however, it transpires that it is far, far worse - it is the ultimate war machine...