The Heretic's Apprentice is a medieval mystery novel by Ellis Peters set in June 1143. It is the 16th novel in the Cadfael Chronicles and was first published in 1989. A returning pilgrim from the Holy Land brings a dowry gift and mind fresh with ideas, leading to charges of heresy and a murder in his household, at the time for celebrating the translation of Saint Winifred's bones to Shrewsbury Abbey. Brother Cadfael, Abbot Radulfus, and Sheriff Hugh Beringar work together to find the murderer, and more difficult, the motive for murder. Bishop Roger de Clinton and an Augustinian canon of the Archbishop of Canterbury bring out the challenge of deciphering true religious belief from heresy, on the border with Wales in the midsummer days of 1143.