Act I: Giulio Cesare in Egitto takes place over a six-month period in BCE 48 to 47, after Caesar defeated the Roman general Pompey at Pharsalia in Greece and pursued him to Egypt, where Pompey has sought refuge with Ptolemy and Cleopatra, the country’s brother-and-sister joint rulers. (We’ll now continue with their Italianate names used in the opera.) Cesare is welcomed on his arrival in Alexandria by Pompeo’s wife, the noble Cornelia, and her son, Sesto. He is also greeted by Tolomeo’s general Achilla, who bears a horrifying gift: the head of Pompeo, whom Tolomeo has ordered to be slain. Cornelia and Sesto are devastated by the sight, as is Cesare, who had been planning to settle a truce with his old rival and now sends a message back to Tolomeo expressing his contempt and disgust at this brutal deed. Cleopatra, too, is shocked at Tolomeo’s treachery and decides to ally herself with Cesare. Tolomeo and Achilla, meanwhile, decide to kill Cesare for his insult.

Disguising herself as Lidia, the Queen’s handmaiden, Cleopatra visits Cesare and uses her formidable seductive powers to draw him to her side against Tolomeo. Cesare is promptly smitten. Cornelia voices her grief at the murder of her husband, while Sesto promises to revenge them both by killing Tolomeo. Tolomeo invites Cesare to stay at the royal palace, but Cesare, distrusting his hospitality, remains wary. Sesto challenges Tolomeo to a duel, but the Egyptian king sends him to prison while punishing Cornelia by making her work in his seraglio. After a moving duet, the two are separated.

Act II opens with a scenically and musically gorgeous interlude, in which Cleopatra, still known to him as Lidia, successfully completes her seduction of Cesare. She invites him to a garden in the palace, where she appears as the goddess of Virtue in a beautiful set depicting Mount Parnassus and the nine Muses of antiquity, who accompany her bewitching song, “V’adoro, pupille.” When the besotted Cesare rushes toward her, she and the set disappear. But they soon meet again in the palace, where their lovers’ tryst is rudely interrupted by Curio’s announcement that a mob is outside, attempting to kill Cesare. Cesare rushes off to gather his followers and defeat Tolomeo’s soldiers. Having finally revealed her real identity to Cesare, Cleopatra is left to grieve his loss and pray for his safety.

The scene moves to Tolomeo’s seraglio, where the Egyptian king is one of many suitors importuning the beautiful Cornelia. He chooses her as his concubine for the night, but is thwarted by the arrival of Sesto, who has escaped from prison and now attempts to seize his father’s sword and slay him. He is unsuccessful. Word now arrives that Tolomeo’s forces have routed Cesare’s and that Cesare has jumped into the harbor and apparently drowned. With these multiple cliff-hangers, Act II closes.

Act III: The numbers against Tolomeo are building. Achilla was also a serious suitor of Cornelia and was pushed out of the way by Tolomeo’s lust. Feeling betrayed, he takes his troops to Cleopatra to use against Tolomeo. But again, Tolomeo’s soldiers defeat those of Achilla. Cleopatra is taken prisoner by her brother and put in chains. She also believes Cesare is dead and sings of her grief in the aria “Piangerò.”

But Cesare is not dead. In an actual historical event, he has swum across Alexandria’s harbor but now is separated from his troops. Sesto and Nireno have also arrived near where he came ashore and have found the dying Achilla, who gives Sesto a seal allowing him to lead Achilla’s troops. Overhearing them, Cesare grabs the seal from Sesto and hurries off to rescue Cleopatra. He urges her to rouse her followers and meet him at the harbor.

In the royal hall, Cornelia defends herself from Tolomeo with a dagger. Sesto rushes in and slays the perfidious king, fulfilling the revenge the pair had planned. In a joyous finale at the harbor, Cesare and Cleopatra celebrate their victory over Tolomeo and proclaim their undying love. Cornelia hands Tolomeo’s crown and scepter over to Cesare, and he proclaims Cleopatra sole ruler of Egypt. She, in turn, declares she will be a tributary monarch to Rome. All rejoice.