'Claudius the God' is actually part two of a two-part set, the second volume after the much-better-known 'I, Claudius'. The story is set in Rome at the time of the institution of Augustus, the first emperor, up to the accession of Nero, the last of the Julio-Claudian line of emperors (after this time, the imperial seat was more of a political prize to be fought for than a family bequest).
Robert Graves intriguing use of the vernacular language and the extensive research, following largely the histories of Suetonius (a gossipy historian) rather than Tacitus (the formal, more official historian), gives a rather racy and juicy insight into the flamboyant lifestyle of the early imperial family, as seen through the eyes primarily of its most unlikely heir, Claudius the stammerer. Claudius escaped much of the political intrigue and was seen as a harmless outsider due to his physical impediments, which helped mask his intellectual capabilities and cunning insight into the actions of others.
Grave's recreation is well-done, but a bit too sympathetic to his hero Claudius. Claudius was not the intellectual saintly character protrayed in theses novels--true, he wasn't nearly as bad as his predecessor Caligula or his successor Nero, but he had shortcomings that are often ignored for lacking the glamour of the evils of the two emperors who bookend his reign.
Graves' use of language is interesting to note. Instead of translating historical scenes into formal, high-academic English (as a classically-trained Oxbridge scholar might be inclined to do), he put things into what Alistair Cook called the everyday language of the English aristocracy, a social class accustomed to the easy exercise of world-domination power, politically and socially. This makes it an engaging work that avoids the pitfalls of academic histories.
This particular volume looks at the later part of Claudius' life, concluding with the time when he was emperor. The intrigues that had carried on in the royal family continued unabated around him as emperor, except that the wise and almost omniscient Claudius of old seemed to develop blind spots once in the imperial seat, largely due to love. When his wife Messalina finally plots his overthrow with a divorce, he acts, and his life is rather sorrowful ever after.
Derek Jacobi's performance in the BBC production is stunning; what the novel leaves out in way of historical accuracy to detail (Claudius was married more times than would Graves' books attest, for instance) it more than makes up for by way of being an entertaining introduction to imperial Rome. Make sure to get both volumes!