In So Great a Light, So Great a Smoke, Louisa A. Burnham takes us inside the world of a little-known heretical group in the south of France in the early fourteenth century. The Beguins were a small sect of priests and lay people allied to (and sharing many of the convictions of) the Spiritual Franciscans. They stressed poverty in their pursuit of a Franciscan evangelical ideal and believed themselves to be living in the Last Days. By the late thirteenth century, the leaders of the order and the popes themselves had begun to discipline the Spirituals, and by 1317 they had been deemed a heresy. The Beguins refused to accept this situation and began to evade and confront the inquisitorial machine.

Burnham follows the lives of nine Beguins as they conceal themselves in cities, construct an "underground railroad," solicit clandestine donations in order to bribe inquisitors, escape from prison, and venerate the burned bones of their martyred fellows as the relics of saints. Their actions brought the Beguins the apocalypse they had long imagined, as the Church's inquisitors pursued them along with the Spirituals and began to arrest them and burn them at the stake. Reconstructing this dramatic history using inquisitorial depositions, notarial records, and the previously unknown Beguin martyrology, Burnham vividly recreates the world in which the Beguins lived and died for their beliefs.