Ten years of war are at end. Troy lies in ruins. The Greeks, led by Agamemnon, have returned triumphant though exhausted. They have lost many of their greatest heroes, including Achilles. In the aftermath of war, thousands of Trojans have been taken prisoner, including Andromaca, widow of Hector, and their son, Astianatte. The two are being held in the town of Epirus by Pirro, son of Achilles and ally of Greece. The Greeks, terrified that the son of a Trojan hero still lives, have already tried to execute the child, but he was saved by Pirro who, despite his engagement to the Grecian princess Ermione, loves Andromaca.

The opera opens with a rare choral overture, preempting the first scene where the captive Trojans mourn the fall of Troy. Andromaca embraces her son and rejects Attalo’s reminder that she could set him free by returning her captor’s love. As mother and son are separated, the scene shifts to Ermione’s followers, who try to distract Ermione from her terrible jealousy and anger at Pirro’s desertion. Pirro arrives and the two defiantly confront one another. Oreste, who loves Ermione deeply, arrives and Ermione rejoices that she will at last be avenged, to which Pirro replies with further defiance.

Oreste tells Pilade how he is tortured by his unrequited love for Ermione, and his friend attempts to calm him. Pirro’s court gather to hear Oreste, an important ambassador from Greece, and, as expected, he asks for Astianatte to be handed over to the Greeks. Pirro vows to make Astianatte his own heir and asks Andromaca to marry him. The chorus foretells the start of a new war, but Andromaca remains adamant that she will never marry Pirro.

Ermione meets Oreste, and the two bemoan their ill-fated loves. The chorus announces that Pirro has changed his mind and will return to Ermione. The finale grows to a climax of conflicting emotions.

The act begins with Attalo’s announcement to Pirro that Andromaca has at last given in to his wishes. The two meet, and while Pirro rejoices, Andromaca secretly vows to kill herself as soon as she has ensured her son’s safety by the marriage. Ermione finds Andromaca bewailing her fate and accuses her of ensnaring Pirro. Andromaca refuses to be drawn into a quarrel and leaves Ermione alone in her agony.

In Ermione’s gran scena, the heroine goes through the gamut of emotions. First she begs Fenicio to bring Pirro to hear her one last time. Sorrow and submission turn to fury as the wedding march of her rival is heard. She calls to Oreste to avenge her dishonor. As he leaves in terrible confusion to carry out her wishes, she invokes the gods to guide her avenger. There follows a brief scene in which Fenicio and Pilade exchange their fears for their countries’ fates, after which Ermione describes the deadly conflict of her emotions. Oreste returns and the Greeks, incensed at Pirro’s marriage to an enemy, have carried out his revenge for him and Oreste announces the king’s death. Ermione, nearly out of her mind, revokes her call for revenge and banishes Oreste from her sight. Pilade and the Greeks drag him away, pursued by the people of Epirus, while Ermione accuses him of deserting her, and falls senseless to the ground.