Part One
Bradamante, disguised as her brother Ricciardo, and her tutor Melisso have come to Alcina’s island in search of Bradamante’s love, Ruggiero. They meet Morgana, Alcina's sister, who falls in love with the disguised Bradamante and promises to take them to the Queen. Alcina is revealed, surrounded by her court, including Ruggiero. Bradamante confronts Ruggiero, who recognizes her as Ricciardo but denies any interest in Ricciardo's sister—he is the faithful lover of Alcina. He runs off to find her. Oronte, rightly suspecting the constancy of Morgana, whom he loves, charges Bradamante with having stolen Morgana's love. Morgana defends Bradamante and insults Oronte. Bradamante tries to calm their mutual recriminations. Oronte, coming upon Ruggiero, who is sighing for the absent Alcina, decides to alleviate his own jealousy by making Ruggiero jealous too, and concocts a tale that Alcina now loves "Ricciardo" and will no doubt soon add Ruggiero to her collection of discarded and transformed lovers. Ruggiero believes him and heaps reproaches on the puzzled Alcina, who assures him that her feelings are unchanged. Bradamante accuses Ruggiero of disloyalty, but he retaliates with defiance, accusing her, as Ricciardo, of having stolen Alcina's love. Bradamante discloses her identity, but Melisso, worried that Ruggiero is not yet ready for this information, convinces him that she is not really Bradamante. Morgana warns Bradamante that Ruggiero has persuaded Alcina to change her into a wild beast, so Bradamante tells Morgana to assure Ruggiero that she does not love Alcina, but rather Morgana herself...
Melisso, disguised as Ruggiero's tutor Atlante, reproaches him with having abandoned the path of glory and gives him a magic ring that brings him to his senses. Ruggiero regrets his faithlessness to Bradamante and wishes to send a message of defiance to Alcina, but Melisso advises him to pretend that he still loves her and make his escape on the pretext of going hunting. Bradamante again reveals her identity, only to have Ruggiero reject this revelation as another of Alcina's deceptions. Alcina is preparing to change Ricciardo into a wild beast to appease Ruggiero, while Morgana tries to dissuade her and Ruggiero assures her that he is no longer jealous, so drastic measures are no longer necessary. Alcina notices that Ruggiero is not in his usual spirits and he suggests a hunt as a restorative. She consents and he departs. Oronte brings the news that Ruggiero is planning to flee. The heartbroken Alcina prepares to foil this plan.
Intermission
Part Two
Oronte tells Morgana that her new love is about to leave her, but she refuses to believe this and departs scornfully, leaving him to lament her power over him. Ruggiero is at last convinced that Bradamante is really herself. Morgana discovers them embracing, reproaches her for being a faithless guest and Ruggiero with betraying Alcina. Ruggiero looks forward to the ending of the enchantments. Alcina begins to make spells to bind Ruggiero to her, but realizes her powers have deserted her. Morgana tries to ingratiate herself with Oronte, who pretends indifference, but has to admit to himself that he still loves her. Alcina upbraids Ruggiero for trying to leave her. He tells her that his betrothed Bradamante now has his love. Alcina threatens vengeance, though unable to obliterate her tender feelings for him. Bradamante and Melisso join Ruggiero to plan their campaign. Melisso tells them that the island is surrounded by Alcina's enchanted monsters. Although worried at leaving his beloved, Ruggiero sets off, followed by Melisso and Bradamante, who vows to free those lying under enchantment. Oronte announces to Alcina the complete defeat of her forces and she laments her cruel fate.
Ruggiero and Bradamante confront Alcina, each advising the other not to be taken in by her deceptions. She tries both pleading and threats in vain. Ruggiero smashes the orb which holds Alcina's secret power. All her spells are broken, and all are transformed. Ruggiero, Bradamante and Melisso leave the broken hearted Alcina alone with her grief.