A new case for Sister Fidelma in 7th century Ireland. A scholar is found dead in his cell in an abbey. However, the door was locked from the inside, there are no other entrances, and the only key lies next to the body. All his manuscripts have disappeared. The abbot sends for Sister Fidelma. She is to solve the case that threatens to shake the foundations of the Irish Church and becomes embroiled in a dark intrigue surrounding a mysterious document.