Comprising a cycle of linked stories set in Dorset, England, it depicts a 20th century in which the Catholic Church still has supremacy; in its timeline, Protestantism was destroyed during wars that resulted from the aftermath of the assassination of Queen Elizabeth in 1588.

As a consequence, while Spanish power eventually wanes, the Catholic Church has no rivals and the pope becomes the effective secular, as well as spiritual, ruler over Europe.

The social effects include a continuing feudal system and bans on innovation, particularly electricity, leading to a roughly mid-19th century technology with steam traction engines and mechanical semaphore telegraphy.

Outlying areas are dangerous, with wild animals and occasional manifestations of the "Old Ones" or "People of the Hills" (supposed fairies) who leave crab-symbol graffiti.

Real geographical locations play a major role: Golden Cap is the site of a semaphore station, and the castle at Corfe is a key presence in the book.