Astianatte sleeps in a prison in the palace after the destruction of Troy. Pirro wants to marry Andromaca, widow of Hector. If she agreed and stopped mourning her dead husband, they will free her son Astianatte. Ermione, Pirro's soon to be wife, gets jealous and when she confronts Pirro, they argue. The arrival of Oreste is announced. He is in love with Ermione and is reminded by Pilade of his duties. Oreste, representing all the kings of Greece, tells Pirro that Astianatte must die before he is able to avenge the death of his father Hector. Pirro defies Oreste, who warns him that the Greeks will be angry with him. Ermione's love for Pirro has turned to hate but when Oreste declares his love to her, she rejects him. Pirro announces that he has changed his mind and will return to Ermione and hand over Astianatte to him. As the guards prepare to do so, Andromaca pleads with Pirro to give her time to think. When Ermione gets furious, Pirro rejects her again but Andromaca declares she will commit suicide if she has to marry Pirro.
Non proseguir! Comprendo!, duet between Ermione and Pirro
Reggia abborrita!, Oreste and Pilade's cavatina
Andromaca agrees to marry Pirro who orders to release Astianatte. But Andromaca wants to kill herself before making her vows at the altar. Pirro appears in the gallery to the sound of a festive march, and he and the wedding procession pass by. Ermione, jealous, wants revenge and gives a dagger to Oreste to kill Pirro. But later on, she is unsure about her feelings and she regrets ordering Oreste to kill him. Pirro is, in fact, attacked and killed, but not by Oreste, who says that he had given the dagger to another man and then reclaimed it. Astianatte is released and Ermione realises that she was still in love with Pirro and asks the Eumenides to destroy Oreste. Pilade and his sailors arrive, telling Oreste to flee with them while Ermione curses him.