Place: London, England
Time: 1601, during the reign of Queen Elizabeth I

Robert Devereux, 2nd Earl of Essex, favorite of Queen Elisabeth, has been removed from office as Governor of Ireland because, acting on his own initiative, he has agreed to a ceasefire with the rebels. Following an attempted uprising, he is awaiting his trial for high treason in London.

Act 1
Scene 1: The Great Hall at Westminster

Sara, Duchess of Nottingham, is trying in vain to hide her tears from the eyes of the Court, as she reads the sad story of Fair Rosamond, the unfortunate lover of King Henry II of England, and therein recognizes a very similar situation to her own. She is in love with Robert Devereux, her husband's closest friend. The Ladies of the court express concern, but she replies that she is happy, while privately revealing her sadness (All'afflitto è dolce il pianto). Elizabeth enters and states that, at the insistence of Nottingham, she has agreed to see Robert once again, now that he has returned from Ireland accused of treason (Duchessa..Alle fervide preci). The Queen is willing to release him without charges if she can be sure of his continued loyalty. To Sara's gradual dismay, the Queen reveals her love for Robert (L'amor suo mi fe' beata). Cecil enters and announces that Parliament is waiting for an answer from the Queen regarding the charges against Robert, since it considers her as being too lenient towards him, but she refuses to sign the death warrant proposed by the Royal Council.

Robert enters and, in a conversation overheard by the increasingly distraught Sara, Elizabeth declares her love for him. Now alone together, Elizabeth promises Robert that the ring she once gave him will always be the pledge of his safety should he ever return it to her. The dream of bygone happy days is shattered by an inappropriate comment by Robert, who assumes that Elizabeth knows the secret of his love for Sara. The Queen, increasingly jealous, demands that Robert name the woman he loves. He denies that he loves anyone (Nascondi, frena i palpiti), and then the Queen leaves.

Nottingham, Robert's friend and supporter, enters and the two men discuss Robert's situation and Nottingham's concerns about his wife's behaviour after he has observed her embroidering a blue shawl (Forse in quel cor sensible, Qui ribelle ognum ti chiama). The two men are interrupted by Cecil demanding that Nottingham attend a meeting of the Peers of the Realm.

Scene 2: Sara's Apartments at Nottingham House

Sara is alone when Robert enters, declaring her to be faithless because she has married Nottingham while he was in Ireland. She defends herself saying that it was the Queen's idea and that she was forced to do her bidding. At the same time, seeing the ring on Robert's finger, she assumes it to be a love token from the Queen, and tells him that they must never see each other again, giving him the blue shawl as a love token. In a final duet (Dacchè tornasti, ahi misera) each declares love for the other and they accept that they must say goodbye. Robert makes plans to escape.

Act 2
The Great Hall at Westminster

The Queen approaches Cecil to find out what has been decided. Cecil declares that the sentence is death. The Queen, asking Raleigh why the whole process took so long, learns that Robert had a shawl in his possession which he resisted giving over. It is handed to her. Nottingham enters and pleads for Robert's life (Non venni mai si mesto), insisting that he is innocent, but the Queen continues to describe how she knows that Robert has been unfaithful and, when he is brought in, confronts him, showing him the scarf. Nottingham sees it as well and recognizes it. Furious, he declares that he will have vengeance; while at the same time, Elizabeth offers Robert his freedom if he reveals the name of her rival. He refuses and she signs the death warrant, announcing that a cannon shot will be heard as the axe falls. Nottingham fumes that the axe is not a suitable punishment.

Act 3
Scene 1: Sara's Apartments

Alone, Sara receives Robert's ring along with a letter from him. In it, he tells her to take the ring to Elizabeth and beg for mercy. Before she can leave, Nottingham arrives and reads the letter (Non sai che un nume vindice). Although she protests her innocence, he prevents her from leaving. They both hear the funeral march for Robert as he is led to the Tower, and Nottingham leaves to exact his revenge on Robert. She faints.

Scene 2: The Tower of London

In his cell, Robert ponders as to why it appears that his ring has not been received by the Queen. But he refuses to betray Sara (Come uno spirto angelico... Bagnato il sen di lagrime), and when Cecil arrives at the door of the cell, it is not to free Robert but to take him to his execution. He is led away.

Scene 3: The Great Hall at Westminster

Elizabeth is mournful about the pending death of her lover and wonders why Sara is not there to give her comfort (Vivi ingrato, a lei d'accanto). Cecil announces that Robert is on his way to the block, and Sara arrives disheveled. She gives Elizabeth the ring along with confessing her guilt at being the Queen's rival. In vain, the Queen tries to stop the execution, but they hear the cannon announcing Robert's death. After Nottingham has arrived, Elizabeth demands to know why he prevented the ring from being brought to her. He replies: "Blood I wanted, and blood I got!" Elizabeth is haunted by the headless corpse of Robert, and longs for her own death, announcing that James VI of Scotland (son of Mary Queen of Scots) will be king. Alone, she kisses Robert's ring.