14 AD Augustus dies. The step son of Augustus, Tiberius, inherits the empire. He loses no time in displaying his political acumen and both offends the senate and provokes the legions in Pannonia and Germany into revolt. Tiberius's nephew, Germanicus, puts down the revolt and keeps the troops busy by leading them on an extended campaign through Germany - which achieves nothing of note at the end of the day, except to make Germanicus terribly popular with the legions. 19 AD Germanicus dies with distasteful celerity.
People are suspicious.
Fingers point and tongues wag and their owners get to join Germanicus rather sooner than they might have. Tiberius heads off for an extended break in his neat little villa on the beautiful island of Capri. Power is left in the hands of a trusted subordinate - a commander of the Praetorian Guard by the name of Sejanus, a hard working arriviste who is working rather harder at it than Tiberius supposed. On Capri Tiberius decides that the best PR he can give himself for posterity is to provide an heir to the empire who would be so awful that even Tiberius would look good in comparison. Luckily Gaius Caligula is on hand.