When analysis of a small piece of cedar wood from a dig indicates that it is 2,000 years old and originates from the Holy Land, Dr Gillian Magwilde and her team race to uncover the truth. While Magwilde surveys the site, however, a religious extremist acquires the land and declares it Holy Ground. Believing Britain is at war for its Christian soul, the fanatical Christian reawakens a force of violent modern-day crusaders.

The high-risk exploration of secret Celtic chambers beneath the Roman Baths unlocks an ancient love story. Gillian and Ben unravel the tragic tale of Celtic warrior queen Boudicca and Roman soldier Quintanus, while confronting their own past relationship amid the dark and claustrophic chambers.