What is the main conflict in Claudius the God and His Wife Messalina by Robert Graves?

Told from the point of view of the stuttering, physically deformed Tiberius Claudius Drusus Nero Germanicus (most commonly referred to as "Claudius,"), I, Claudius covers the reigns of Augustus, Tiberius and Caligula, and ends at the point of Claudius himself reluctantly assuming the position of emperor shortly following Caligula's assassination.