Part I

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Julius Caesar enters Alexandria with his general Curio and his army to be met by Cornelia and Sesto, the wife and son of Pompey, who have come to sue for peace. Tolomeo’s general Achilla arrives with a greeting and gift from his king: Pompey’s severed head. Cornelia laments her loss and Sesto swears to avenge his dead father. In the palace at Alexandria, Cleopatra plots to take sole possession of the throne. When she learns of Tolomeo’s betrayal of Pompey from his servant Nireno, she resolves to visit Julius Caesar’s camp and win his favour and aid. Achilla tells Tolomeo of Julius Caesar’s angry reception of his gift and offers to accomplish his murder if Tolomeo grants him Cornelia as a reward. Tolomeo fears the growing strength of Rome under the leadership of Julius Caesar and agrees. At the camp, Julius Caesar presides over Pompey’s funeral rites. In disguise, Cleopatra presents herself to Julius Caesar as Lidia, a handmaiden in the palace, but one of noble birth. Captivated, Julius Caesar promises to help her. Cleopatra and Nireno watch as Cornelia pays her last respects to her dead husband. Cornelia and Sesto plan to kill Tolomeo. Cleopatra comes forward and, though still disguised as Lidia, pledges the support of the Queen of Egypt, offering them the services of Nireno as a guide into the palace. Cornelia and Sesto confront Tolomeo. He is struck by Cornelia’s beauty, but when Sesto challenges him to combat, he orders their imprisonment. 

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Part II

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In the palace, with Nireno’s help Cleopatra has arranged an entertainment calculated to ensnare further the interest of Julius Caesar. Sesto who has escaped with the aid of Nireno, who brings the news that Tolomeo has commanded she become one of his concubines. He will help to smuggle Sesto into the harem as well, where, unguarded, the king will be at his most vulnerable. In her apartment, Cleopatra waits for Julius Caesar, pretending to be asleep when he enters. The lovers’ tryst is interrupted by Curio, who has overheard Achilla’s assassin searching the palace for Julius Caesar. Cleopatra reveals her true identity to Julius Caesar and begs him to fly to safety. Julius Caesar refuses and goes to meet his enemy. Cleopatra prays to the gods to preserve him and the love she now truly feels for him.

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Part III

Sesto steals into the Harem and tries to use Tolomeo’s sword to kill him but is disarmed by Achilla. He summons the king to war; Julius Caesar is believed to have plunged to his death in the harbour during the previous night’s assault, but Cleopatra has fled to the Roman camp and has there mustered an army to march on the palace. Tolomeo prepares to lead his troops, but refuses Achilla the promised reward for Julius Caesar’s death; Cornelia is to be his own prize. A furious Achilla decides to switch his allegiance to Cleopatra. In the ensuing battle, Tolomeo’s troops are victorious and Cleopatra is taken prisoner. Julius Caesar, though, is not dead; the sea has cast him up onto the shore. Sesto and Nireno stumble across the wounded Achilla dying on the beach. Overheard by Julius Caesar, he gives Sesto a seal, the sight of which will give the bearer authority to command his troops. As Achilla dies, Julius Caesar takes the seal from Sesto and goes to assemble a force to storm the palace. In prison, Cleopatra has prepared to take her own life. Julius Caesar breaks in with his troops and rescues her. Believing himself to be victorious, Tolomeo sees no obstacle to his pursuit of Cornelia, but Sesto at last seizes his chance and kills him. The battle has been won and Julius Caesar crowns Cleopatra as sole Queen of Egypt. She declares her allegiance to the Roman Empire. The lovers sing and all welcome the arrival of peace.