In order to prevent the capture of Jerusalem by the knights of the First Crusade, The Prince of Darkness has sent the enchantress Armida into the world to seduce the Christian heroes and turn them from their duty. The bravest of these, Rinaldo, has fallen under Armida's spell. She comes to love him so deeply that she cannot bring herself to destroy him.
Scene 1: A council chamber in the royal palace of Damascus. Idreno is alarmed that the Crusaders have crossed the river Jordan. Armida, the heathen sorceress seems to have triumphed over the Christians but feared that her conquest was not complete without gaining the love of the Christian knight Rinaldo. Now Rinaldo is obsessed with Armida and promises to fight against his fellow Christians. If victorious King Idreno offers him the kingdom and Armida's hand. Armida prays for Rinaldo's safety.
Scene 2: A steep mountain, Armida's fortress at the top. Knights Ubaldo and Clotarco plan to free Rinaldo from Armida's clutches. Idreno sends Zelmira, the daughter of the Egyptian sultan, to ensnare the Christians but on encountering Clotarco she falls in love with him and offers to lead him to safety.
Scene 3: Armida's apartments. Rinaldo admires the bravery of the approaching knights. Ubaldo warns Rinaldo to beware Armide's charms, and reproaches the dereliction his duty as a Christian. Although remorseful Rinaldo is unable to escape Armida's enchantment.
Scene 1: A garden in Armida's palace. Zelmira fails to dissuade King Idreno from planning an ambush on the Christians. Idreno pretends to agree to Clotarco's demand that the Christian knights enchanted by Armida be freed. Reluctantly, Rinaldo leaves with Ubaldo. Armida expresses her fury.
Scene 2: The Crusaders' encampment. Ubaldo welcomes Rinaldo who prepares to go into battle. Armida begs for refuge and Rinaldo's love. Rinaldo departs for battle with Ubaldo and the other soldiers.
Scene 1: A dark, forbidding grove, with a large myrtle tree. Rinaldo, knowing that the tree holds the secret of Armida's powers, enters the wood intending to cut it down. Zelmira and nymphs try to get him to return to Armida. As he is about to strike the myrtle Armida, dishevelled appears from it and confronts him. Armida cannot bring herself to kill him; Rinaldo strikes the tree and the magic wood vanishes.
Scene 2: The crusaders' encampment. The crusaders prepare for battle against the Saracens. Armida appears swearing to pursue Rinaldo everywhere. As Rinaldo moves off she sends an infernal chariot after Rinaldo.