Outside Jerusalem, during the Crusades. The paladins (knights of the Crusades) are eager to take to the battlefield. Their commander, Goffredo, arrives and reminds them they must first perform the funeral rites for Dudone, their recently deceased leader. A noblewoman appears and introduces herself as the rightful ruler of Damascus. She claims that her throne has been usurped by her evil uncle Idraote and asks for help and protection. In fact she is the sorceress Armida and in league with Idraote, who has entered with her, disguised as her attendant. Their plan is to weaken the Crusaders by enslaving some of their best soldiers. The men are so dazzled by Armida’s beauty that they convince Goffredo to help her. Goffredo decides that the paladins must choose a new leader, who will then pick ten soldiers to go with Armida. They elect Rinaldo, much to the jealousy of the knight Gernando (“Non soffrirò l’offesa”). Armida and Rinaldo, who is the Crusaders’ best soldier, have met once before and she is secretly in love with him. She now confronts him and reminds him how she saved his life on that occasion. When she accuses him of ingratitude, he admits that he’s in love with her (Duet: “Amor… possente nome!”). Gernando sees them together and insults Rinaldo as a womanizer in front of the other men. They duel and Rinaldo kills Gernando. Horrified by what he has done and yet convinced his comrades will see that he was only protecting his honor, he decides to stand his ground. But when the soldiers turn against him, he escapes with Armida, who conjures up a storm to distract the Crusaders.
Astarotte, one of the princes of hell, has led a group of demons from the underworld to a ghastly forest. He instructs them to respond to a sign from Armida by building an illusory pleasure palace. Armida arrives with Rinaldo, who’s completely enthralled by her (Duet: “Dove son io!”). Even when she tells him about Idraote’s plot, he doesn’t turn against her. To Rinaldo’s amazement, Armida then seems to turn the forest into a vast palace. Various entertainments are offered to Rinaldo: songs and dances of nymphs, Armida’s own song on the power of love (“D’Amore al dolce impero”), and a ballet that reflects Armida and Rinaldo’s own story and her hopes for its outcome. Rinaldo, losing all thoughts of military honor, gives himself over to Armida’s enchantment.
Two of Rinaldo’s fellow knights, Ubaldo and Carlo, have been sent on a mission to rescue him. When they arrive in Armida’s enchanted gardens, they are overwhelmed by their beauty, even though they know it’s all an illusion. With the help of a magical golden staff, they ward off the nymphs that try to seduce them, then hide when Rinaldo and Armida appear. Rinaldo is still captivated by the sorceress, but once he is alone, Ubaldo and Carlo confront him. When they show him his reflection in a shield, he’s horrified to realize that he no longer recognizes himself as the honorable warrior he once was (Trio: “In quale aspetto imbelle”). Still torn by his love for Armida, Rinaldo prays for strength. He imagines seeing a celestial light, confirming his decision to escape. When Armida discovers his absence, she calls upon the powers of hell to bring her lover back but her spell doesn’t work. She rushes off in pursuit of the men.
Armida reaches the three soldiers before they can sail away. She begs Rinaldo not to desert her and even offers to go into battle with him. Ubaldo and Carlo restrain Rinaldo, trying to bolster his strength, and ultimately drag him away from her. Two figures, Love and Revenge, appear before Armida, and she struggles to decide what to do (“Dove son io?… Fuggì!”). She chooses Revenge, destroying the pleasure palace and flying away in a rage.