In Part I, David Williams traces the poetics of teratology, the study of monsters, to Christian neoplatonic theology and philosophy, particularly Pseudo-Dionysius's negative theology and his central idea that God cannot be known except by knowing what he is not. Williams argues that the principles of negative theology as applied to epistemology and language made possible a symbolism of negation and paradox whose chief sign was the monster.

Part II provides a taxonomy of monstrous forms with a gloss on each. Part III examines the monstrous and the deformed in three heroic sagas - the medieval Oedipus, The Romance of Alexander, and Sir Gawain and the Green Knight - and three saints' lives - Saint Denis, Saint Christopher, and Saint Wilgeforte. The book is beautifully illustrated with medieval representations of monsters.