Elisabetta has sent her beloved Roberto Devereux, the Earl of Essex, to lead a military expedition to Ireland. Against the Queen’s orders, Roberto has signed a peace treaty with the Irish rebels. Jealous of his favorable position with the Queen, her advisors seize the opportunity to push through a charge of treason against him.
Parliament prepares to discuss charges against Roberto Devereux. Sara, the Duchess of Nottingham and close friend of Queen Elisabetta, is plagued with guilt for having an affair with Roberto. Elisabetta arrives and expresses her love for Roberto, and confides to Sara that she suspects Roberto of being unfaithful. The Queen is interrupted by a visit from Lord Cecil and Gualtiero who convey the Parliament’s frustration over her leniency towards Roberto. To stall the Parliament, she demands more proof of guilt before making a decision. When a page arrives with a request from Roberto (who is under house arrest) to meet with the Queen, her heart is stirred with a hope that their affection will be rekindled and she agrees to the visit.
Elisabetta meets Roberto in private and reminds him of the ring she gave him, a special token that will guarantee his safety if he only sends it back to her. She tries to win his heart by reminding him of their happy days together. Roberto is shocked at Elisabetta’s tenderness towards him and accidentally reveals that he is in love with another woman. Seeing Elisabetta’s initial anger escalate to fury, he tries to deny everything but the damage has been done: Elisabetta is set on vengeance against Roberto and the woman who has won his affection. With Elisabetta infuriated, Roberto’s only remaining ally is the Duke of Nottingham (Sara’s husband). Nottingham discloses to Roberto that he observed his wife sobbing while embroidering a blue scarf and expresses his concern over his wife’s secret suffering. Their exchange is interrupted by Lord Cecil who summons Nottingham on behalf of the Queen to a meeting of the Parliament to decide on Roberto’s sentence. Nottingham vows to save his friend.
Roberto can’t resist a secret meeting with his beloved Sara while Nottingham is out. He berates her for getting married while he was at battle. She explains that after her father’s death, the union was ordered by the Queen to ensure her financial stability. Sara also reminds him of his previous royal affection by pointing to his ring. They acknowledge their mutual love but realize that they can never be together. Roberto pledges his love by giving his ring to Sara as a parting gift and she gives him her blue embroidered scarf in return.
At the hall at Westminster, the Parliament reaches a decision and Lord Cecil informs the Queen of the verdict: the death penalty. Only Elisabetta’s signature is needed for the execution to be ordered. She dismisses the court for a private meeting with Gualtiero, one of her trusted spies. Gualtiero informs the Queen that Roberto was arrested after being out until dawn; when searched, they found a blue scarf hidden under his shirt. Gualtiero produces the scarf as proof of his story. Upon careful examination Elisabetta notices love knots embroidered into the scarf, which confirm her suspicions, and fuels her decision to agree to sign Roberto’s death warrant. Nottingham begs the Queen to have mercy on Roberto. His pleas are silenced when Roberto is brought in and Elisabetta reveals the blue scarf demanding to know its owner. Recognizing the scarf as belonging to his wife, Nottingham is horrified at Roberto’s betrayal. In a fiery jealous rage, the Queen signs the death sentence.
Alone in the tower of London, Roberto reflects on his life of lies and indiscretions with great regret. He remains hopeful that Elisabetta will receive the ring and pardon him. The guards arrive and Roberto allows himself to be led to his execution without protest. Elisabetta has unresolved feelings about signing Roberto’s death warrant. She desperately hopes Roberto will return her ring as a sign of his renewed devotion. To her horror, Cecil enters and informs her that Roberto is being led to his execution. At that exact moment, Sara bursts in and unable to speak, hands the ring to Elisabetta, silently identifying herself as the Queen’s rival. It is too late: a cannon shot is heard signifying the fatal blow has been delivered and Elisabetta lashes out at Nottingham and Sara for their part in Roberto’s death. They are taken into custody and the Queen laments the events that led to her bloodstained crown. Haunted by Roberto’s severed head and the destruction she has caused her subjects, Elisabetta longs for death. The Queen gives up her royal power by declaring King James of Scotland her nephew, heir to the throne, the new King of England.