The magician Zoroastro appears to Orlando in a dream and conjures up an allegorical vision representing the struggle between military glory and romantic love that dominates the hero’s thoughts. Zoroastro, declaring himself to be the guardian of Orlando’s sense of valorous duty, recommends the former, but Orlando (who has fallen in love with Angelica) reflects that even the great heroes of antiquity had their softer moments. He decides to devote his service to chivalry. The shepherdess Dorinda observes Orlando rescuing a beautiful princess from her foes. Struck by the sight of the famous warrior, she is left confused and unable to explain the strange new emotions in her heart. Angelica, Queen of Cathay, secretly awaits her lover, Medoro. While caring for his wounds, she had fallen deeply in love. When he arrives, Angelica offers him her hand and her throne. He declares himself unworthy, but she reassures him. Dorinda now approaches Medoro. She is suspicious of her friend’s constant attention to the beautiful Queen. Medoro quickly replies that it is his duty to accompany the woman responsible for saving his life on her journey back to her homeland, but that he will never forget the sweet Dorinda. She ruefully admits that love tempts her to believe words she knows to be untrue. Zoroastro warns Angelica of Orlando’s vengeance should he discover her with Medoro. However, in his desire to restore Orlando’s energies to combat, he promises to assist the lovers. Orlando approaches his beloved, but Angelica tries to repulse him by accusing him of loving the princess whom he had recently rescued. He denies this and says that he will do whatever she wishes to prove his affection. As Medoro is seen approaching, Zoroastro and his genii produce a fountain that instantly shields the prince from Orlando’s view. Safe for the moment, Angelica continues her ruse by doubting the sincerity of Orlando’s affection. The hero vows to enter the fiercest combat in order to prove his love. Dorinda discovers Angelica and Medoro together again. Angelica admits that Medoro is her spouse and the couple attempt to console the heartbroken shepherdess.
Dorinda sings to a nightingale of her unrequited love. Orlando enters, demanding to know if it was she who spread the false rumor of his supposed affection for the rescued princess. Through her denial, Orlando learns of Angelica’s imminent departure with Medoro, whose beauty Dorinda describes. In his despair, Orlando vows to pursue Angelica and wishes for his own death. Zoroastro exhorts Angelica and wishes for his own death. Zoroastro exhorts Angelica and Medoro to escape from the jealous Orlando. He then offers advice to his apprentices, warning them that when Cupid is one’s guide, all reason vanishes. Medoro carves his and Angelica’s names, entwined, on a tree and then goes to prepare for their departure. Angelica reflects that, although she must distress Orlando (to whom she owes her life), she cannot rule her own heart. Orlando, delirious, enters and attacks her. Zoroastro intercedes, allowing for her escape. Orlando is left devoid of reason and is surrounded by Stygian Furies. In his madness, he sees a vision of Pluto and the weeping Proserpina, who reveal themselves as a mocking Medoro and Angelica.
Medoro comes to Dorinda’s cottage, where he and Angelica had agreed to meet should they be parted. Dorinda receives him kindly, and he regretfully explains that his heart beats for someone else. Orlando enters and in his madness mistakes Dorinda for Angelica and attempts to make love to her despite her protests. He then mistakes her for one of his foes and images himself locked in mortal battle. Angelica learns from Dorinda of Orlando’s plight, and hopes for his recovery. Dorinda reflects on the strange effects of love. Zoroastro enters, changes the scene to a cavern, and announces that Orlando will soon be cured. Angelica now learns from Dorinda that Orlando has destroyed her cottage and buried Medoro in its ruins. Orlando appears, once again threatens Angelica, and hurls her into the cavern. The earth, he says, is now rid of monsters, and he sinks into an exhausted slumber. An eagle descends, bringing a vial of healing elixir, which Zoroastro’s genii sprinkle on Orlando. He wakes into sanity, but, believing that in his madness he has killed Angelica, tries to kill himself. Angelica arrives and restrains him. Orlando declares that he is now victorious over himself, and wishes happiness to Angelica and Medoro. All celebrate his victory in a final chorus.