This work establishes the chronology and provenance of the early mediaeval tales known today as the "Four Branches of the Mabinogi". Although they have been justly described as 'fundamentally the stories of the old Brittonic gods from whom the leading Welsh dynasties claimed descent', which makes their principal subject-matter archaic and in principle timeless, Tolstoy shows that often seemingly incongruous and contradictory passages reflect details of historical events in Britain and Ireland during the first two decades of the eleventh century. It is not suggested that the author of the "Four Branches" was attempting a pastiche history of the times in which he lived, but rather that he envisaged historical events as reflections of themes drawn from the mythology of pagan Britain. Dramatic events culminating in the battle of Clontarf in Ireland and the Danish conquest of England in the same year were viewed by the author as echoing those gormesoedd, Otherworld scourges which had assailed Britain in illo tempore, as Mircea Eliade termed the concept. In order to substantiate this central argument, Tolstoy's researches range widely across mediaeval history, in the course of which he investigates and provides fresh answers to many perennial problems which have long exercised the minds of scholars of the period.