Ireland AD 670. When an eminent scholar is found murdered in his cell in the abbey of Lios Mór, fear spreads among his brethren; his door was secured from the inside, with no other means of exit. How did the murderer escape? And what was the content of the manuscripts apparently stolen from the scholar's room? Abbot Iarnla insists on sending for Sister Fidelma and her companion Brother Eadulf to investigate the killing.

But even before they reach the abbey walls, there is an attempt on their lives. As the mystery deepens, Fidelma and Eadulf must also wrestle with problems of their own, which threaten to separate them for ever...