Act 1
General Pompey has lost to Julius Caesar and taken refuge in Alexandria with Tolomeo, Pharaoh of Egypt. Caesar follows Pompey to Alexandria. Pompey's wife and son, Cornelia and Sextus, beg Caesar to pardon Pompey, and Caesar agrees. Tolomeo's general Achilla (hoping to ingratiate Tolomeo with Caesar) presents Caesar with the severed head of Pompey. Caesar is horrified; Cornelia collapses, and Sextus vows revenge. Tolomeo's sister Cleopatra wishes to wrest power from him and rule as sole queen of Egypt. She plots to visit Caesar's camp in disguise and seduce him.
Tolomeo is warned by Achilla that Caesar was disgusted by the killing of Pompey. Achilla offers to kill Caesar if Tolomeo will give him Cornelia. Tolomeo agrees. Cleopatra, escorted by her attendant Nirenus, arrives in Caesar's camp. Cleopatra claims to be Lydia, "handmaiden to Cleopatra." Tolomeo has robbed her of her fortune, she says. Caesar, immediately smitten, promises to help and withdraws. Cornelia and Sextus arrive to mourn Pompey. Cleopatra, still disguised as Lydia, offers to help them gain access to Tolomeo.
Sextus challenges Tolomeo to a duel. Tolomeo arrests mother and son, ordering that Sextus be sent to prison and Cornelia be made to tend the harem gardens. Achilla declares his love to Cornelia, promising that she and Sextus will be freed if she yields to him. She refuses. She and Sextus say farewell.
Act 2
Cleopatra has prepared a spectacle for Caesar. Nirenus leads Caesar to her. Still posing as Lydia, Cleopatra brings him completely under her spell, then disappears. Nirenus offers to lead Caesar to Lydia's room. Cornelia once again rejects Achilla. Tolomeo enters, and Achilla, not realizing that Tolomeo is now in love with Cornelia himself, assures him that the plans to assassinate Caesar are well underway. When Achilla leaves, Tolomeo tries to make love to Cornelia, and when she refuses him, he threatens force. Alone, Cornelia resolves on suicide.
Sextus, released from prison by Nirenus, arrives just in time to stop her. Sextus again vows to avenge his father's murder. Caesar proclaims his love to "Lydia." His general Curio comes to warn him that assassins are looking for him. Cleopatra reveals her true identity. Emboldened by her revelation, Caesar tells her that he must now leave to fight for her and defeat the conspirators. Cleopatra prays for his safety, realizing that she has come to love him.
Act 3
Tolomeo's army defeats Cleopatra's forces and Cleopatra is taken prisoner. Caesar, who has escaped death by swimming away, has been washed ashore. On the same shore, Sextus and Nirenus find the mortally wounded Achilla, who confesses to having engineered Pompey's death. He gives Sextus a seal that will allow him to command Achilla's men, tells them of a secret entrance to the palace, and dies. Caesar approaches and takes the seal, vowing to rescue Cleopatra and Cornelia.
Surrounded by Tolomeo's guards, Cleopatra bids farewell to her handmaidens. Caesar appears and releases her. Elsewhere in the palace, Tolomeo forces himself upon Cornelia, but Sextus appears. They fight, and Sextus kills Tolomeo. Caesar crowns Cleopatra Queen of Egypt. She names herself Tributary Queen to Caesar. They reaffirm their love for each other, as all present sing, "Let joy return to our hearts."