Julius Caesar has just defeated his rival Pompey in Alexandria, where Cleopatra and her younger brother Ptolemy are currently vying for the throne of Egypt. Pompey had appealed to Ptolemy for arms and refuge, but Ptolemy betrayed him in an attempt to gain Caesar’s favor. Cornelia and Sesto, the wife and young son of Pompey, come to Caesar and the Roman general Curius to beg for peace. The entire group is horrified when Achilla, Ptolemy’s general, reveals Pompey’s severed head on a golden platter. Caesar condemns Ptolemy’s barbarous act, and Sesto and Cornelia vow to have vengeance.

When she hears of her brother’s actions, Cleopatra resolves to use this as an opportunity to win Caesar’s favor for herself. Meanwhile, Achilla informs Ptolemy of Caesar’s anger. The two make a deal: if Achilla murders Caesar, Ptolemy will give him Cornelia as a reward.

Cleopatra meets Caesar in his camp, disguised as Lydia — a servant to Cleopatra, born of noble blood, whose fortune was stolen by Ptolemy. Caesar is enchanted by her beauty and promises to settle her case in court. Later, “Lydia” overhears Cornelia and Sesto discussing their revenge and promises them Cleopatra’s support.

Caesar meets Ptolemy and the two exchange tense niceties, each while plotting the other’s demise. Ptolemy invites Caesar to stay in his palace, and Caesar accepts. When Caesar exits, Sesto challenges Ptolemy to a duel. Instead, Ptolemy imprisons both Sesto and Cornelia. After Ptolemy leaves, Achilla promises to free them if Cornelia agrees to marry him. Cornelia is horrified and Sesto tries to protect her, but Achilla separates them and sends Sesto off to prison.

Cleopatra invites Caesar to her palace for entertainment. She has Nirenus, her servant, tell Caesar that Lydia will wait for him at sunset. Meanwhile, Cornelia weeps hopelessly in her imprisonment but still refuses to marry Achilla. She tries to flee from him, only to run into Ptolemy, who also desires her. Sesto, who escaped from prison with the help of Nirenus, saves his mother from ending her life in despair. Nirenus informs them that Ptolemy intends to make Cornelia one of his concubines, but proposes that they use this as an opportunity to finally seize their revenge.

In the garden, Caesar proclaims his love to Lydia. Curius interrupts them to tell Caesar that Achilla’s assassins are searching for him. In her distress, Cleopatra accidentally reveals her true identity to Caesar and vows to defend him with all her might. When Achilla’s men breach her palace, Cleopatra advises Caesar to flee, but he refuses to back down and runs toward the battle.

Ptolemy is relaxing in his harem, savoring Cornelia’s beauty amongst his other concubines. Sesto is hiding in the harem and attempts to steal Ptolemy’s sword, but Achilla rushes in and stops him. Achilla then reports that his soldiers have overwhelmed Caesar and Curius, who jumped off a balcony into the sea and are believed dead. Cleopatra is now gathering Caesar’s troops to invade Ptolemy’s camp. Achilla reminds Ptolemy of his promise to award Cornelia for his services, but Ptolemy refuses to keep that promise. Enraged, Achilla plots to betray Ptolemy and join Cleopatra’s service.

After a fierce battle, Ptolemy’s troops defeat Cleopatra’s, and he takes her prisoner. Cleopatra spits defiance at Ptolemy, who vows to tame her pride. Meanwhile, Caesar has survived the fall into the water and made it safely to shore. Sesto and Nirenus find Achilla dying on the beach, holding a seal that allows the bearer to command his troops. Caesar steals the seal from Sesto and leaves to rescue Cornelia and Cleopatra. When he bursts into her chambers, Cleopatra is overjoyed to see her lover alive and well.

Ptolemy still believes he has won the day. He strengthens his attempts to seduce Cornelia; she draws a dagger from her garment to avenge her dead husband. In that same moment, Sesto arrives and slays Ptolemy. Cleopatra is crowned Queen of Egypt, and she declares her alliance with the Roman Empire.