After the overture, the entire cast, except Giulio Cesare, gathers on stage for the opening chorus. (Chorus: Viva, viva il nostro Alcide). Giulio Cesare and his victorious troops arrive on the banks of the River Nile after defeating Pompeo’s forces. (Aria: Presti omai l’Egizia terra).
Pompeo’s second wife, Cornelia, begs for mercy for her husband’s life. Cesare agrees, but on the condition that Pompeo must see him in person. Achilla, the leader of the Egyptian army, presents Cesare with a casket containing Pompeo’s head. It is a token of support from Tolomeo, the co-ruler of Egypt (together with Cleopatra, his sister). Cornelia faints, and Cesare is furious about Tolomeo’s cruelty. (Aria: Empio, dirò, tu sei).
Cesare’s assistant, Curio, offers to avenge Cornelia, hoping that she will fall for him and marry him. Cornelia rejects the offer in grief, saying that another death would not relieve her pain. (Aria: Priva, son d’ogni conforto). Sesto, son of Cornelia and Pompeo, swears to take revenge for his father’s death. (Aria: Svegliatevi nel core).
Cleopatra decides to use her charm to seduce Cesare. (Aria: Non disperar, chi sà?) Achilla brings the news to Tolomeo that Cesare was furious over the murder of Pompeo. Tolomeo swears to kill Cesare to protect his rule of the kingdom. (Aria: L’empio, sleale, indegno). Cleopatra (in disguise) goes to meet Cesare in his camp hoping that he will support her as the queen of Egypt.
Cesare is amazed by her beauty. (Aria: Non è si vago e bello). Nireno notes that the seduction was successful. (Aria: Tutto può donna vezzosa). Meanwhile, Cornelia continues to mourn the loss of her husband. (Arioso: Nel tuo seno, amico sasso). Cornelia prepares to kill Tolomeo to avenge Pompeo’s death, but is stopped by Sesto, who promises to do it instead. Cesare, Cornelia and Sesto go to the Egyptian palace to meet Tolomeo. (Aria: Cara speme, questo core). Cleopatra now believes that as she has turned Cesare, Cornelia and Sesto against Tolomeo successfully, the scales are tipped in her favour. (Aria: Tu la mia stella sei). Cesare meets Tolomeo, who offers him a room in the royal apartments, though Cesare tells Curio that he expects Tolomeo to betray him. (Aria: Va tacito e nascosto). Tolomeo is fascinated by Cornelia’s beauty but has promised Achilla that he could have her. (Aria: Tu sei il cor di questo core). Sesto attempts to challenge Tolomeo, but is unsuccessful. When Cornelia rejects Achilla, he orders the soldiers to arrest Sesto. (Duet: Son nata a lagrimar).