Armide's two confidantes Phenice and Sidonie are celebrating her recent victory over the crusaders and trying to distract her from her preoccupation with the idea of defeating Renaud, the most valiant crusader of them all. Armide describes how she has recently seen him in a dream in which she fell in love with him at the very moment he struck her a fatal blow. Hidraot, King of Damascus and Armide’s uncle, accompanied by his retinue, enter. He congratulates Armide on her victory over the crusaders but urges her to marry. She confesses that, if she marries, it will only be to the man who defeats Renaud. The people of Damascus celebrate Armide's victory with singing and dancing. Their rejoicing is interrupted when Aronte staggers in, wounded, telling them that their prisoners have been rescued by one soldier, single-handed --Renaud. They all swear vengeance on him.
Renaud is with Artemidore, one of the knights he has rescued, who is on his way back to the camp of the crusaders. He himself cannot do so because he has been banished by their leader, Godefroi. Artemidore warns him to beware of Armide, but Renaud declares his indifference to the danger she poses.
As they leave, Armide and Hidraot come in and invoke the spirits of the underworld to put a spell on Renaud. When Renaud appears, he is entranced by the beauty of the countryside. Evil spirits transformed into naiads, nymphs, shepherds and shepherdesses lull him to sleep with songs and dances. Armide suddenly appears, ready to stab the sleeping Renaud. But she cannot bring herself to do so. She determines to make him love her, thereby avenging herself by seducing him from the field of battle. In shame and desperation she calls on her spirits to take them both far away to the end of the universe.
Armide reflects that although, by means of her magic, she has made Renaud fall in love with her, she has failed to control her own amorous feelings for him. Phenice and Sidonie assure her that she has him completely in her power, but she knows she must choose between her love for Renaud and hatred and revenge on him. Left alone, Armide summons Hate to rescue her and exorcise love from her heart.
Hate and her demonic entourage appear from the underworld and begin their incantations. But this is too much for Armide and, her mind made up, she joins in, rejecting Hate's help. Hate scornfully predicts Armide's abandonment, saying that she could not punish her more harshly than by leaving her to the powers of Love.
The Danish Knight and Ubalde, sent to Armide's palace to find Renaud and bring him back to the war, find their way barred by Armide's monsters. They overcome these with the help of a talismanic diamond shield and a golden scepter given them by a magician to counter Armide's magic powers.
The desert changes into beautiful countryside and the two knights are in turn tempted by demons, which have been transformed by Armide to resemble their beloveds, Lucinde and Melisse. They dispel these visions with the golden scepter and go on their way towards Armide's palace.
Armide is now committed to love Renaud but fears she will lose him, as Hate predicted. Armide goes to consult her evil powers and leaves Renaud to be entertained by her retinue until she returns. They take the form of Pleasures who sing and dance for him in a lengthy divertissement, but Renaud cannot enjoy anything without Armide, and banishes the Pleasures. Ubalde and the Danish Knight enter and show Renaud the diamond shield. This breaks Armide's spell and awakens him to his desertion. As he is leaving with his companions, Armide returns and expresses her horror at seeing her fears of his betrayal confirmed.
After the knights have left, Armide alternates between despair and desire for vengeance. She laments the departure of the perfidious Renaud and then regrets she did not kill him when he was in her power. Finally she orders the demons to destroy her enchanted palace, and as they burry her love in it forever, she leaves to seek revenge on Renaud