The mutual love of the Princess Ginevra and Prince Ariodante has full approval of her father, the King of Scotland. As the opera begins, she is confiding her feelings to Dalinda when Polinesso, the Duke of Albany, who covets the throne, bursts into her room and makes advances to her, which she forcefully rejects. Dalinda tells him that Ginevra’s heart is already bestowed, but confides that she herself is not indifferent to him: a plot thereupon forms itself in his mind to make use of her to be revenged on Ginevra and gain his ends. Meanwhile, in the royal gardens, Ariodante and Ginevra, exchanging vows, are given the blessing of the King, who intends making Ariodante his successor.

Polinesso persuades Dalinda to dress up as Ginevra and admit him into her room that night: to convince Dalinda to aid him in this deceit, he promises to respect her honor and make her his wife. Dalinda is overjoyed, and when Ariodante’s brother, Lurcanio, appears and declares his love for her, she quickly evades him. The act ends with the betrothed royal couple expressing their happiness and calling upon the nymphs and shepherds to celebrate their joy in dance and song.

That night Ariodante, unable to sleep for excitement, is walking in the royal gardens when he encounters Polinesso, who feigns surprise at the news of the forthcoming marriage and claims to be enjoying Ginevra’s favors. When Ariodante furiously reaches for his sword at these outrageous words, Polinesso promises to substantiate his charge: he tells him to hide and observe with his own eyes. Lurcanio, who has been surprised to see his brother talking with the disliked Polinesso, has also concealed himself and is watching events. Polinesso knocks on the secret door to the royal apartments, and in answer to his signal, Dalinda, disguised as Ginevra, lets him in and closes the door. Ariodante, horrified at this apparent betrayal, is about to kill himself by falling on his sword when Lurcanio, who has also been duped, rushes forward to prevent him throwing away his life for a worthless woman. Ariodante goes off in utter despair, while Polinesso, swearing devotion to Dalinda, gloats over the success of his ruse.

The following morning the King is in council, about to declare Ariodante his heir, when Odoardo brings a report that the Prince has thrown himself into the sea in a sudden frenzy and has been drowned. The King hurries to break the news to Ginevra, who collapses in shock. His own grief is heightened when Lurcanio, accusing Ginevra of unchaste behavior that drove his brother to his death, demands justice in the lists: he will fight anyone who offers to champion her cause. The King disdainfully declares that a wanton is no longer his daughter. Ginevra, bewildered at the charge and at his rejection, goes out of her mind.

Polinesso has hired two assassins to silence Dalinda. They set about her in a forest, but Ariodante, who has been wandering about aimlessly and dejectedly, chances to be there and beats them off. She is amazed to see him alive, and he is equally astonished to learn from her of the trickery of which he was the victim, and which she now sees involved in her own death. He sets out immediately for the palace with her.

The King has refused even to see his daughter until a champion for her cause can be found. Polinesso, with an eye to succession to the throne if he is successful, offers himself; though Ginevra refuses his aid, her father insists on his acceptance. Polinesso meets Lurcanio in public combat and is felled by a mighty blow. Lurcanio, still burning to avenge his brother’s supposed death, challenges any further champions of Ginevra, and the King himself is about to enter the lists to retrieve his family honor when a knight, whose face is hidden by his closed visor, appears for her defense. Lurcanio tells him to prepare for combat, but the knight raises his visor and, to general astonishment, reveals himself as Ariodante. He offers to explain all if the King will pardon Dalinda for her unwitting part in the deception; Odoardo brings news that Polinesso, as he lay dying, has confessed his treachery. The King at once hastens to his daughter with the happy news. Dalinda, repenting her former love as well as the deception into which it led her, now gladly accepts Lurcanio’s renewed wooing.

Ginevra, in the apartment to which she has been confined, is giving way to despair when the King joyfully arrives to tell her she is vindicated; he frees her, embraces her, and reunites her with Ariodante. She is astonished and enraptured to find him alive after all, and the opera ends with general rejoicing in the great hall of the palace.