Act One

Princess Ginevra is in love with Ariodante, a knight. Her father, the King of Scotland, approves of her choice and accepts Ariodante as his son-in-law and heir; Polinesso, the Duke of Albany, would also like to marry Ginevra and succeed to the throne, but Ginevra spurns his suit.

Dalinda, a lady of the court who attends on Ginevra, is infatuated with Polinesso and rejects Ariodante's brother, Lurcanio, who loves her dearly. Polinesso decides to use Dalinda in his plot to get rid of Ariodante, his rival for the throne. He promises to court Dalinda if she will help him by dressing in Ginevra's clothes and letting him into the princess's chambers one night.

Act Two

Ariodante tells Polinesso about his forthcoming marriage to Ginevra and Polinesso, pretending to know nothing of the engagement, declares that Ginevra has on several occasions favoured him with unchaste tokens of love. When Dalinda, dressed as Ginevra, now appears as arranged and goes off with Polinesso, Ariodante believes the lies the duke has told him. He is beside himself with despair at this apparent betrayal by his fiancee and wants to commit suicide. Lurcanio, who has also observed what has taken place, restrains him, urging him not to throw away his life on a worthless woman; if anyone deserves to die it is she.

The king is about to declare Ariodante his heir when Odoardo, a courtier of the king, comes in with the news that Ariodante has thrown himself into the sea. Lurcanio accuses Ginevra of having been the cause of his brother's death by being unfaithful. The king is not prepared to acknowledge a wanton as his daughter and Ginevra is totally bewildered. She cannot understand the accusations levelled against her and almost goes out of her mind, is plagued by nightmares and longs for death.

Act Three

Ariodante has survived his attempted suicide. He saves Dalinda from two assassins whom Polinesso has hired to murder her. From Dalinda Ariodante then learns the truth about the duke's intrigue.

The king insists that a knight be found to champion Ginevra's cause in a trial by ordeal against Lurcanio. Polinesso offers himself. As he believes Ariodante to be dead, there would be nothing to stop him from marrying Ginevra if he were victorious. Polinesso and Lurcanio fight and Polinesso is mortally wounded by Lurcanio and confesses his guilt as he lies dying.

Ariodante returns to the king's court and explains everything, announcing Ginevra's innocence and begging the king to pardon Dalinda for the unwitting part she has played. Lurcanio assures Dalinda that he still loves her dearly and urges her to love him back, and after some hesitation she consents. The king publically acquits Ginevra of all guilt and she is reunited with Ariodante.