The action takes place in Edinburgh, Scotland in the late-eighth century.
ACT I: In her dressing room in the royal palace, The Princess Ginevra is making herself beautiful for the man she loves (Vezze, lusinghe — Charms and adornments). She tells her friend and confidant, Dalinda, she is in love and her father, the King, approves of the match. Polinesso, the Duke of Albany rather imprudently enters and declares his passion for her, but she indignantly rebuffs him and leaves. Dalinda, who is in love with Polinesso, informs him that his rival is the knight, Ariodante, and advises him to find someone else, hinting at her own love for the Duke. (Apri le luci — Open your eyes). Left alone, Polinesso devises a plot to overthrow his rival by using Dalinda's love for him (Coperta la frode — If deceit is covered)
In the garden, Ariodante sings of how the countryside speaks of love (Qui d'amor). Ginevra and Ariodante pledge their love for each other in the duet Prendi da questa mano (Take from this hand). The King gives them his blessing and instructs Odoardo to prepare for a wedding (Voli colla sua tromba — Fly with her trumpets). Alone, Ariodante thinks of his happiness (Con l'ali di costanza — With the wings of constancy).
Polinesso asks Dalinda to dress as Ginevra that evening and conduct him to the chamber of the Princess. If she does, his heart will be all hers (Spero per voi — I hope for you). When he leaves, Lurcanio, Ariodante's brother, enters and tells Dalinda he loves her, but she says she is not for him (Del mio sol vezzosi rai — The lovely rays of my sun). Alone, she declares she will be ever faithful to Polinesso.
Ballet: Ariodante and Ginevra meet in a lovely valley where they are entertained by shepherds and shepherdesses.
ACT II: By a ruin from which a door leads to the apartments of Ginevra, Polinesso meets Ariodante and pretends to be amazed when he is told Ariodante and Ginevra are to be married. Overheard by Lurcanio, Polinesso, tells Ariodante he himself is the object of Ginevra's love and offers to prove it. If he is lying, Ariodante says, Polinesso will die. Polinesso knock on the door which is opened by 'Ginevra' (Dalinda in her friends clothes) who leads him inside. The distraught Ariodante tries to kill himself, but Lurcanio stops him and urges him to seek revenge. Alone, Ariodante bemoans his fate in the recitative E vivo ancora? (And I still live?) and aria Scherza infida (The unfaithful woman amuses herself).
Dawn approaches as Dalinda and Polinesso bid farewell; he tells her she will hear only lover's sentiments from him. Naïvely she believes him and leaves him to rejoice at the success of his scheme.
As the King is preparing to announce the up-coming wedding, Odoardo tells him that the despairing Ariodante has thrown himself into the sea and is dead. When Ginevra is told, she faints and is carried off. The King tries to comfort Lurcanio on the death of his brother, but he wants only justice, not comfort. He hands the King a letter detailing what he had observed the previous evening and vows he will support his accusations with the sword. As a result, the King denounces his daughter.
Ballet: In her madness,Ginevra imagines a battle between pleasant and sorrowful Dreams.
ACT III: As Ariodante, who survived his suicide attempt, wanders in a wood denouncing the gods for letting him live, he hears cries. It is Dalinda who is trying to escape from assassins set on her by Polinesso to murder the only witness to his deception. Ariodante drives them off, and Dalinda confesses the trick she and Polinesso played. Ariodante hastens to return to court, while Dalinda cries down the wrath of heaven on the man who betrayed her.
Back at the palace, the hypocritical Polinesso offers to be Ginevra's champion in a joust. Under sentence of death, Ginevra is brought to the King. She protests her innocence, begging to be allowed to kiss his hand. Her sorrowing father takes her hand and tells her Polinesso will be her champion. Although she tries to refuse, he insists.
At the jousting grounds, the court watches the fight between Polinesso and Lurcanio. Polinesso is mortally wounded, and Lurcanio offers to fight anyone else who wishes to defend Ginevra. An unknown knight appears with the visor of his helmet down. He is revealed to be Ariodante who promises to tell all if Dalinda is pardoned. Odoardo comes with the news that Polinesso, on his deathbed, has confessed his misdeeds . The King pardons everyone and hurries to find his daughter, followed by Ariodante.
Lurcanio again professes his love for Dalinda, and she admits the man she had loved is a traitor.
As Ginevra, in the prison, is preparing for death, the others arrive and apologize for doubting her.
Ballet: In a Royal Hall, the happy couples are entertained by the dances of knights and ladies of the court as the curtain falls.