It is the year 999 in a remote Benedictine monastery in Italy. Brother Thomas of Worms, highly educated, intelligent, with a thirst for knowledge, suspects murder in the death under suspicious circumstances of one of the monks. He tries to convince the Abbot, but the Abbot insists that it was an accident. Thomas' punishment for his arrogance in disputing the Abbot is to copy a New Testament book: the Revelations of John.

At the same time, young monk Brother Nicholas discovers a text that has been written over. He shares this with Thomas and they discover two rare, ancient Gnostic manuscripts which may undermine the teachings, not just of the Church, but of all Christianity.

Three more monks meet with accidents that Brother Thomas is convinced are not accidents, but murder. Because the year is 999 AD and a new millennium is near, many of the monks fear that evil with a capital E is at hand. Thomas finally convinces the Abbot and receives permission to pursue his inquiries.

To Know Evil gives an intriguing look at life in a monastery in medieval times in northern Italy. The characters are well drawn. The atmosphere in the monastery is a moody, gloomy one. There is an interesting detailed description of the preparation of parchment. The story is fast paced with twists at the end that come as a surprise.