Frazer opens from a single, strange custom reported at the shrine of Diana Nemorensis at Lake Nemi in Italy: the sanctuary's priest, styled the King of the Wood, held office only by having killed his predecessor in single combat, and could himself be killed and replaced by any challenger who first broke a golden bough from a sacred tree in the grove. Rather than treat that custom as a local curiosity, Frazer used it as a thread to pull on, asking what wider pattern of belief could make sense of a priesthood defended by ritual murder, and followed that question outward into an enormous comparative survey of magic, myth and religious practice across cultures and periods far removed from ancient Italy.
The resulting argument rests on two supposed laws of sympathetic magic, imitative magic, where like is thought to produce like, and contagious magic, where things once in contact are thought to remain linked. Frazer read a wide range of harvest rites, fertility customs, taboos around sacred kings and myths of dying and reviving gods, Adonis, Attis, Osiris among them, as expressions of the same underlying magical logic applied to different problems: securing crops, ensuring a ruler's vitality, or renewing the fertility of the land through a god's death and return.
The book's structure, radiating outward from one Italian custom to material gathered from ethnographic reports across the world, is what gave it its distinctive and controversial method.