Tacitus, a Roman senator writing in the early second century, composed the Annals as a year-by-year history of Rome's emperors after Augustus, from Tiberius through Nero. This is Book 4 of that work. The previous books left Germanicus dead and his alleged poisoner Piso dead by his own hand after a Senate trial, while the charge of injured majesty began to punish speech.

Aelius Sejanus, prefect of the Praetorian Guard — the emperor's household troops — gathers them into a single camp at Rome and becomes a second sovereign in all but name. Drusus, Tiberius's son, dies; Livilla, the prince's wife, and the prefect hang over the death. Prosecutions peel friends away from Agrippina, Germanicus's widow. Senators compete in flattery, voting honors to a knight who holds the swords. Tiberius withdraws toward Capri, an island off the Italian coast, ruling by letter and suspicion. Sejanus is not yet fallen. This is the making of a favorite, and of a city living under two tyrannies at once.