Cesare and his troops settle victoriously in Alexandria aſter defeating Pompeo’s forces. Cesare, who knows nothing about the fate of the vanquished enemy, is generous: he agrees to an appeal from Pompeo’s wife and son – Cornelia and Sesto – for a peaceful settlement of his and Pompeo’s old rivalry.
Their joy was tempered, however, by Achille, leader of the Egyptian military, who brings Cesare a casket containing the head of Pompeo, presented as a giſt from the king of Egypt Tolomeo. Cornelia is grief-stricken, she faints as a result. Cesare is furious about Tolomeo’s cruelty and sends Achille back with a message of contempt and disgust. He orders to bury Pompeo’s body with the dignity and the respect that he deserves.
Cornelia tries to kill herself, but suicide is prevented by Curio and Sesto, who 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 Pompeo from her servant Nireno, she resolves to visit Cesare’s camp and win his favour and aid. She and Tolomeo bicker before she steals away. Achille brings Tolomeo the news that Cesare was unhappy with Pompeo’s death, Tolomeo is furious.
He swears to kill Cesare to protect his rule of the kingdom. Achille agrees to accomplish this murder if Tolomeo grants him Cornelia as a reward. Tolomeo fears the growing strength of Rome under the leadership of Cesare and agrees.
Cesare presides over Pompeo’s funeral rites. Cleopatra presents herself to him disguised as Lydia, a handmaiden in the palace, of noble birth but robbed of her birthright by Tolomeo. Captivated, Cesare promises to help her. Meanwhile, Cornelia pays her last respects to her dead husband. Cornelia and Sesto plan to kill Tolomeo to take revenge. Cleopatra offers to guide them into the palace.
Cesare goes to the Egyptian palace to meet Tolomeo. Cornelia appears and reproaches the king of Egypt in treachery, Sesto vows to avenge the death of his father. In a rage, Tolomeo orders to arrest Sesto and to exile Cornelia to the garden where she will have to take care of the flowers. Contrary to his promise to Achille he is willing to take her as his concubine.
Cleopatra instructs Nirenus to lead Cesare into her room. He arrives and is enchanted by her song. Disguised Cleopatra swears she will love Caesar until the dying day. Cesare is confused,
he can’t marry a handmaiden. Curio interrupts them and reports that armed soldiers come for Cesare. Cleopatra reveals her true identity to Cesare and begs him to fly to safety. Cesare refuses and goes to meet his enemy.
Cleopatra, having fallen in love with Cesare, begs the gods to bless him. Tolomeo, secure in his harem, Achille announces that Cesare (in the attempt to run from soldiers) has jumped from the palace window and died.
Now Cleopatra has fled to the Roman camp and mustered an army to march on the palace. Achille then demands that Tolomeo give Cornelia to him, but Tolomeo refuses.
Cesare’s death boosts Tolomeo’s morale and he prepares to lead his troops.
Sesto is desperate and attempts to kill himself, but he is prevented from doing so by his mother; he repeats his vow to kill Tolomeo. Furious at Tolomeo for being ungrateful to him despite his loyalty, Achille plans to defect to Cleopatra’s side.
Tolomeo’s troops are victorious in the battle and Cleopatra is taken, prisoner. Cleopatra mourns Cesare.
However, Cesare is not dead. He survived his leap and is roaming the desert in search of his troops.
In prison, Cleopatra has prepared to take her own life. Cesare breaks in with his troops and rescues her. Now it’s time to deliver the death blow.
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 Cesare 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.