The discovery of a hitherto overlooked Authurian story, buried in a longer poem, gives the author a precise geographical location for the Grail's original hiding place in Britain - the White Abbey, an Augustan priory in Shropshire. Futher research reveals that the Grail was one of several treasures, also including the Golden Bull of Knossos and the sacred candelabra removed from the temple of Jerusalem, which were taken in 1410 AD from Rome to the safety of Viroconium - modern day Shrewsbury - virtually the last vestige of Roman civilization anywhere in the former Western empire.
Phillips takes a cluster of myths, legends, beliefs and tenuous historical connections that form the basis of the Grail stories. He then uses this to determine how they are based on a factual event which is the Grail arriving in England. With all these works there is always a high level of assumptions about what is factually true. For example there is a convent close to a historical site important to his thesis. Even though it is a couple of hundred years out, does not matter as it is highly probable the site has been reused for the same purpose over and over since the Romans arrived. That is where you question the rationale in the arguments and that is one of many examples. Phillips does solid research in examining all the various theories about Arthur and the Grail and adds his into the mix.