Act 1

Four Roman virtuous senators—Sabinus, Silius, Cordus, and Arruntius—lament the wickedness of Caesar Tiberius and his favorite, Sejanus, a nobody. They remember the days of the republic with nostalgia.

The Senate requests permission to build a temple to Tiberius. The latter affectedly dislikes the flattery and rejects the request. At the same time, he approves the Senate’s decree to set up a statue of Sejanus in Pompey’s theater.

Drusus Senior, a Roman politician and military commander, is exasperated by this sign of favor to Sejanus. He threatens Sejanus, and Sejanus decides to take revenge.

Act 2

Sejanus wants to poison Drusus and conspires with Livia (wife of Drusus Senior, daughter-in-law of Tiberius), whom he wants to marry.

Sejanus and Tiberius talk about Agrippina, Germanicus’s widow, and her friends. Sejanus tries to persuade Tiberius that she, her sons, and their friends pose a threat to Caesar. According to Sejanus, it would be wise to kill some of them to remove the threat. Meanwhile, it is reported that Drusus Senior has been poisoned and is dying.

Act 3

Tiberius feigns deep sorrow over Drusus’s death. Sejanus asks Tiberius for Livia’s hand, but Caesar refuses. He refers to Sejanus not being a patrician and the union being politically inexpedient.

Sejanus contemplates persuading Tiberius into abandoning himself fully to his lusts, hoping, meanwhile, to take control of the government. As for Tiberius, he is apprehensive of Sejanus’s attempts to usurp power. Thus, he decides to set prefect Macro against Sejanus in order to undermine the latter’s position. In his soliloquy, Macro speaks of his ambition and reveals his unscrupulousness.

Act 4

Agrippina is fearful for Nero, Drusus Junior, and Caligula as Sejanus’s power grows.

Macro advises Caligula to go to Tiberius and tell him that Sejanus is scheming against him. Arruntius and Lepidus (another senator) lament the death of Sabinus and deplore the growth of Sejanus’s power. Drusus Junior has been taken prisoner and Agrippina banished. Arruntius accuses Tiberius of depravity and cruelty.

Act 5

Sejanus’s backers tell him of foreboding omens and try to persuade him into bringing sacrifices to the gods. Sejanus rejects the warnings, saying that he worships Fortune alone. Macro plans to arrest Sejanus at a special meeting of the Senate.

At the meeting, a letter from Tiberius is read to the Senate. Caesar mixes praise and criticism of Sejanus in it, and the senators try to glean a hidden message in Tiberius’s words. In conclusion, the emperor demands that Sejanus be deposed, suggesting to the Senate that the issue of confiscation of his property should be discussed, and intimates that justice may demand his life as retribution. Macro, however, does not wait—he immediately orders that Sejanus be executed. Arruntius prophesies that Macro will soon become even more corrupt than Sejanus:

I prophesy, out of the senate’s flattery,
That this new fellow, Macro, will become
A greater prodigy in Rome, than he
That now is fallen.