As the elderly Claudius visits the lavatory, he recalls the reign of Tiberius. He was bitter and corrupted by power, kept in check only by Germanicus. Until . . .
Germanicus has been poisoned. His ashes are carried through the streets of Rome. His family and the people of Rome are devastated and Agrippina incites the crowd against Livia and Tiberius. This worries the Emperor but Sejanus reassures him that his power is secure. Livia warns him that Agrippina and her friends want to try Piso and Plancina on charges of murder and treason for the death of Germanicus. She wants them cleared but Tiberius just wants to hush the matter up.
Agrippina, Herod, Castor and Claudius discuss their strategy for obtaining justice. Agrippina tells an elaborate tale of how Germanicus was haunted by witchcraft and eventually poisoned by Plancina, the wife of Piso, his lieutenant. Strange events afflicted their house in Antioch, including the loss of his green charm.
They are interrupted by the arrival of Caligula, Germanicus' son, who is having strange dreams. Agrippina relates that the Syrians made a great fuss over him.
Agrippina's allies have Martina, a poisoner, as a witness against Piso. Claudius suggests trying the case in the Senate, to avoid corruption.
Piso and Plancina meet with Tiberius. Piso is defiant but Tiberius is distant, which concerns the pair, who are counting on his support for their murder of Germanicus. After Tiberius steps out, Piso shows Plancina letters from the Emperor supporting his actions.
The trial begins. Castor conducts the prosecution. He alleges that Piso killed Germanicus and led the troops in rebellion against the new governor.
Antonia finds Caligula and his sister Drusilla naked together. Claudius tries talking to Caligula, which prompts his mother to wish that he had died instead of Germanicus. They are interrupted by Herod reporting that Martina has disappeared.
Piso defends himself, saying that he was bitter about his improper removal as governor and tried to reinstate himself after Germanicus' death. The Senate are skeptical but Piso says "I had my instructions" and lays down several sealed letters on the table. The Senate asks him to read them out but he can't break the Imperial seal. They ask Tiberius to allow them to be read but he refuses.
A confident Piso returns home. Plancina is worried that they've angered Tiberius but Piso defends himself, saying they shouldn't even be on trial. They fight over who decided to kill Germanicus. Sejanus arrives and tells them that since the letters are state papers, he will take them to the archives. A nervous Piso turns them over.
Sejanus advises Tiberius to abandon Piso to the crowd to protect himself. He tells him the Romans are praising Agrippina above all. Tiberius tells Sejanus to find Martina so that they can convict the pair.
However, Martina is with Livia. They discusses poisoning and Martina details the murder of Germanicus on Plancina's orders. She tells Livia that Caligula helped her with the withcraft. The young boy thought he was a god and wanted to prove this by frightening his own father to death.
Piso's attorney, in the absence of Martina, moves to dismiss the case. When Tiberius refuses, Plancina demands a separate trial. Piso is furious but she says they've been abandoned. She urges him to kill himself to save the family. But he tells her to go to Livia and tell her that he has unsealed letters from her he will reveal if he's not acquitted.
Livia confronts Tiberius but he refuses to acquit the pair. If the letters are revealed, he'll claim Livia is senile. She then tells Plancina that unless she can get Piso to kill himself, she'll produce Martina as a witness.
Plancina offers to die with Piso, not telling him about Martina. He starts but decides to call Tiberius' bluff, which provokes Plancina to stab him to death and take the letters back to Livia.
Agrippina and her allies lament that with Piso's death, their efforts to unseat Tiberius are done. Agrippina worries about her sons. Shortly after, they have to flee - Caligula has set fire to the house.
The Emperor Claudius wakes up and finds his candle has ignited his robe. He pats out the fire and goes back to writing his history.