The Praetorian guard was essentially the secret service, but soon became king makers. The leadership of the guard was sick of being humiliated by emperor Caligula and did not like the way things were heading. So they assassinated Caligula and murdered his family.

The problem was that Caligula's family were the Julio Claudians, descendants of Julius Caesar (the now god), and the people of Rome weren't willing to accept a leader that wasn't of the royal family.

Luckily for them, they found goofy uncle Claudius hiding behind the curtains. Uncle Claudius was the son of Germanicus, one of Rome's greatest heroes, and uncle Claudius had kept himself alive by playing up his disability for the amusement of nephew Caligula (many believe Claudius had cerebral palsy). But now Claudius was the only male descendant of Caesar, so the guards hoisted him on their shoulders and declared him emperor against his will. The Praetorian thought he'd be easy to control, but instead he gutted them and killed the men who declared him emperor.

He became a pretty solid emperor, very much a scholar. And there's a fantastic series called I, Claudius about him.