Act 1

Goffredo, commander of the Christian forces, comforts and rallies the Frankish soldiers, who are mourning the recent death of their 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 in disguise. 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 Franks 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, had 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, he escapes with Armida before Goffredo can punish him.

Act 2

Astarotte, one of the princes of Hell, has led a group of demons into a forest to help Armida. She arrives there 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 turns the forest into a vast pleasure palace. Armida muses on the power of love ("D'Amore al dolce impero") and offers for Rinaldo's entertainment a pantomime about a warrior being seduced by nymphs. Rinaldo, having lost all thoughts of military honor, gives himself over to Armida's enchantment.

Act 3

Two of Rinaldo's fellow knights, Ubaldo and Carlo, have been sent on a mission to save him. When they arrive in Armida's enchanted gardens, they are overwhelmed by the beauty of them, 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, then leaves with his comrades. Armida calls upon the powers of Hell to bring her lover back but finds herself helpless. 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. Armida struggles between love and desire for revenge ("Dove son io?... Fuggì!"). She chooses revenge, destroying the pleasure palace and flying away in a rage.^[5]