Zeno's travels take him from his native Bruges as far away as the court of Gustav Vasa, whose ill-fated son Erik he tutors and attempts to admonish, to the Louvre of Queen Catherine where he encounters the poisoner Cosimo Ruggeri, and also to the lands of the Ottoman Empire, for whose navy he designs a rudimentary flamethrower.

When his young assistant Cyprian is caught in an ilicit Adamite cult, Zeno's true identity is exposed and he is condemned to be burned at the stake for, among other things, the crime of atheism.

The novel is set principally in Flanders of the 16th century, in the period opening the Early modern era of booming capitalistic economy, of renewed approaches to sciences, of religious upheavals and the bloody counter-Reformation, to the background of incessant wars between countries and the creeping chaos of the Black Death.

In Yourcenar's own words, "In alchemical treatises, the formula L'Oeuvre au Noir, designates what is said to be the most difficult phase of the alchemist's process, the separation and dissolution of substance.

"[1] The English title The Abyss gives a slightly different lead by the evocation of fathomless depths, a likely image of the alchemist's inner journey, which are at the same time a Christian vision of hell, to which his contemporaries may wish to condemn him.