The first novel ended with the assassination of Nero and with Claudius being unexpectedly and reluctantly elevated to Emperor.
Claudius was born handicapped and undersized and weak and stuttering, so he remained mostly overlooked during his pre-Emperor years, avoiding the (often fatal) power struggles exercised by his family. So, he seems completely unprepared and unqualified for his role as supreme leader. Despite his initial reluctance, Claudius embraces the role, implementing his policies to modernize the Empire and sometimes executing his enemies without trial. During his 13-year reign, he successfully undoes much of the damage caused by the mad Caligula. Overall, he performed better than almost anyone expected.
At first glance, this seems like a good deal for Claudius.
But his life is damaged by his relationships with women - particularly with Messalina - his young and beautiful wife - who conspires against him and is unfaithful with literally hundreds of other men. After four marriages, Claudius remains unhappy.
Once again, Graves does a good job of bringing to life the political intrigue and personal dramas of ancient Rome and her key players. Nearly everyone in this world with ambition is assassinated or plots assassinations or both. Claudius tries to stay above this and is mostly successful for most of his life.
Previous Roman Emperors had embraced their role as god-king; but Claudius resisted this idea, insisting that he was not a god. In fact, he dreams of Rome eventually rejecting its recent incarnation as a dictatorship and returning to a Republic. His childhood friend the Hebrew king Herod Agrippa takes a different approach, declaring himself to be the prophesied Messiah and is ultimately killed for his arrogance.
But, Claudius changes with time, as happens to so many with ultimate power. Near the end of his life, Claudius accepted his deification when he discovered the Britons were building temples to him. He begins plotting the succession of the throne - without the best interests of the Empire.
The full title of this book is "Claudius, the God and his wife, Messalina" and Claudius's full name is Tiberius Claudius Drusus Nero Germanicus. Much like these names, Robert Graves shortens and simplifies the 13-year reign of Emperor Claudius.