Act I

Caesar’s forces have defeated those of his Roman rival Pompey, and he claims Egypt. Cornelia pleads with him for peace, and for the life of her husband Pompey – but she is interrupted by the Egyptian Achilla, who offers a tribute gift from Tolomeo: the head of Pompey. Caesar scorns Achilla. Cornelia urges her young son Sesto to take revenge on his father’s killers.

Hearing of her brother Tolomeo’s ill-received gift, Cleopatra taunts him. She resolves to seduce Caesar in order to use him to wrest power from her brother. To win his trust, she pretends to be a commoner called Lidia. In the same disguise, Cleopatra also forms an alliance with Cornelia and Sesto, offering her agent Nireno as a guide.

Caesar chides his host Tolomeo for his misconduct and advises him on proper authority. He evades assassination.

When they confront Tolomeo with his treachery (he had previously allied himself with Pompey), Cornelia and Sesto are condemned by him to the harem and to prison respectively. Privately, Achilla offers to help the friendless pair in return for Cornelia’s love. She treats him with contempt. Left alone with her son for a moment, they lament their bleak future.

Act II

Cleopatra has contrived an elaborate scene in which to advance her seduction of Caesar.

In contrast, in the harem Cornelia is subject to the crude advances of Achilla and then of Tolomeo himself. Tolomeo offers to reward Achilla with Cornelia if he succeeds in assassinating Caesar. Cornelia would take her own life, but is prevented by Sesto – who, with the help of Nireno, intends to murder Tolomeo.

Cleopatra pretends to be asleep as she waits to meet Caesar. Making love to her as Lidia, Caesar is shocked when she offers to marry him. Curio brings word of a popular uprising against the Romans, and Caesar in particular. Before he can leave, Lidia reveals herself to be Cleopatra, and urges him to flee rather than fight. Impetuously, he declines. Alone, she is surprised to find that she loves him, and devastated to know that she has lost him and her gamble for power.

In the harem, Cornelia understands that she is Tolomeo’s chosen companion for the night. Sesto plans to take him by surprise, but Achilla barges in, urging Tolomeo to join the fight against the Romans. Achilla explains that Cleopatra’s forces have gone over to the Romans. He asks for assurance of his price for loyalty – Cornelia – but is haughtily dismissed by Tolomeo. When they leave, Sesto’s failure edges him toward suicidal despair. Cornelia prevents him; there is a chance that with Nireno’s help they may yet avenge Pompey’s betrayal and murder.

Act III

Achilla resolves to join Cleopatra’s forces, reacting against Tolomeo’s treachery.

Cleopatra is her brother’s prisoner. With Caesar assumed dead, and Cornelia and Sesto disabled, she has no hope of help from the Romans.

Caesar survives – separated from his men in a desperate naval battle, he is carried by the current to a remote shore. Despairing, he overhears the dying words of Achilla, offering to Nireno and Sesto the seal which will secure the loyalty of his remaining force. Caesar obtains the seal from Sesto, and goes back into action.

Cleopatra prepares to die, but is rescued by Caesar.

Cornelia plans to murder Tolomeo when he tries to rape her. Instead, Sesto kills the defenceless Tolomeo. Sesto is at last the son his mother wanted.

As Caesar is acclaimed lord of the world and Rome’s emperor, Cleopatra declares herself his tributary queen. They proclaim mutual love, and welcome peace. Egypt is subdued by Rome.