The Sister Fidelma mysteries follow Fidelma of Cashel, a seventh-century Irish religieuse and qualified dálaigh, or advocate of the Brehon courts. Working most often with the Saxon scholar Brother Eadulf, she investigates murders, disappearances and political conspiracies across the Five Kingdoms of Ireland and in Northumbria, Rome, Wales, Francia, Brittany and Italy. Her authority derives from law and evidence rather than ecclesiastical rank, allowing the cases to explore a society in which women could hold legal office and rulers remained answerable to established codes.
Beginning at the Synod of Whitby in 664 and continuing into 673, the sequence places criminal investigations within disputes between Celtic and Roman Christian practice, rival dynasties, regional rebellions and the survival of older religious customs. The individual mysteries can be read independently, but the series also develops Fidelma and Eadulf's partnership, marriage and family life while tracing the pressures placed on the kingdom of Muman and the changing Christian world of early medieval Europe.