In 1648 as England toppled into a second civil war the parliamentary army was busy subduing Royalist uprisings and a Scottish invasion. Cromwell sent his top agent, Luke Tremayne, to Ireland to investigate the murder of a fellow officer and the disappearance of a fortune in Spanish silver, and to assist in the defence of Dublin Castle - one of the last remaining Parliamentary garrisons. Disaster strikes before Luke leaves England and once in Ireland his new deputy, Simon Cobb, scarred by the events of the massacre of 1641, adds to the tension. Luke is constantly thwarted by the mysterious Arabella McGregor and by a professional unit of Ulster republicans led by a red headed colonel and a blond deputy. Luke's endeavours are monitored by the amiable Royalist, Sir Gerald Burke and his attractive daughter whose kidnapping leads Luke deep into the Wicklow Mountains. Here he is almost murdered, and uncovers a secret Catholic refuge controlled by an enigmatic Father Ambrose. A sensual herbalist who is accused of witchcraft by the diabolical Abbot Malachy increasingly distracts Luke. The dual mission takes him to the shores of Lough Derg, to Cork and the network of drains under Dublin Castle. The execution of King Charles in 1649 creates great support for the Royalists in Ireland and the army of the new English Republic faces complete annihilation. Luke must find the silver if Dublin Castle is to hold out until the promised arrival of General Cromwell. In a final twist Luke's world is turned upside down.