Milan and the surrounding countryside, early 18th century. Bertarido, king of Lombardy and Milan, has been attacked and deposed by Grimoaldo, an ally of his estranged brother, Gundeberto. Gundeberto was killed in the battle and Bertarido vanished, leaving his queen, Rodelinda, and a young son, Flavio, in the power of the victorious ally, Grimoaldo. As a reward for defeating Bertarido, Grimoaldo was promised the hand of Bertarido’s sister, Eduige—which would grant him a legitimate claim to the throne at Milan. Eduige and Grimoaldo fell in love, but she would not marry him while mourning two brothers—one dead, one presumed so.
From abroad Bertarido has sent word of his own death, intending to return to Milan in disguise, rescue his wife and son, and escape to an anonymous life far from the vagaries of politics and the burden of government. The news of his death has devastated both Rodelinda and Eduige. Grimoaldo, intent on gaining the throne, weighs his options, counseled by two advisers—Garibaldo, his closest aide, and Unulfo, a member of Bertarido’s cabinet who maintains intimate ties with the royal family and is the only person who knows that Bertarido still lives.
Rodelinda and her son are being held in a sparsely furnished room in the palace in Milan. Grimoaldo enters with Eduige and his advisers and announces his wish to marry Rodelinda, thereby gaining the throne. The outraged Rodelinda refuses him and storms away. Eduige is appalled at Grimoaldo’s overture to Rodelinda, but despite the rules of mourning offers him her hand, heart, and throne. Grimoaldo, however, is still stung by her previous postponements and, though still in love with her, fiercely declines Eduige’s offer. Now Garibaldo makes overtures to Eduige, hoping to gain the throne for himself. Eduige, furious with Grimoaldo, does not discourage him. When he is left alone Garibaldo reveals his ambition for the throne.
Bertarido arrives at the stables, where Unulfo has left a soldier’s uniform for his disguise. He finds in the cemetery a memorial built for him by Grimoaldo to appease those loyal to him. Bertarido yearns to see Rodelinda but knows he cannot yet reveal himself. His reunion with Unulfo is interrupted when Rodelinda brings her son to plant flowers at the memorial. Unulfo succeeds in restraining Bertarido, who wants desperately to reach out to his family. Garibaldo appears with an ultimatum from Grimoaldo, to which Bertarido must also be silent witness: either Rodelinda agrees to wed Grimoaldo, or Garibaldo kills the boy. Rodelinda is forced to agree. She takes back her child, lashes out at Garibaldo, and rushes away. Bertarido cannot see past Rodelinda’s surrender to Grimoaldo’s demand. Unulfo promises to find some resolution to the dilemma. Alone and disconsolate, Bertarido grieves over Rodelinda’s seeming loss of faith.
In the palace library Garibaldo again offers his services to Eduige in exchange for her hand—he will kill Grimoaldo if necessary. But he sees from her response that Eduige loves Grimoaldo still. Rodelinda appears with her child and reassures Eduige that her son’s future is her greatest concern. Eduige shares with Rodelinda her confused anger over Grimoaldo’s rejection of her. Grimoaldo enters with Garibaldo and Unulfo, and Rodelinda presents him with an ultimatum of her own: she will marry him on one condition, that he personally kill her son before her eyes. Her gambit works—Grimoaldo backs down; but he is very taken with Rodelinda’s courage and constancy and feels that he might actually come to love her, though he cannot forget his feelings for Eduige. Garibaldo and Unulfo are left alone to debate Grimoaldo’s options. Garibaldo believes power should be seized and ensured at any cost. Unulfo, musing alone, decides to take Rodelinda to Bertarido and finds a breath of hope. Walking near the stables, Eduige happens upon and recognizes Bertarido. She is overjoyed to find him alive. She assuages his fears about Rodelinda’s constancy, and they move away deep in conversation as Unulfo brings Rodelinda to the stables. Unulfo goes off to look for Bertarido, who soon returns with Eduige to be reunited at last with his wife. When they are discovered together by Grimoaldo, he orders Bertarido taken into custody and, enraged, bids them take their final farewells. Bertarido will soon die.
Eduige sends a servant to the dungeon with a concealed weapon that is to be given to Bertarido. She and Unulfo plan for Bertarido’s escape: Unulfo, who has access to the prison, will lead Bertarido through a hidden tunnel from the cell to the palace garden, where Eduige will wait with Rodelinda and the child. From there they will escape. Grimoaldo enters with Garibaldo, who advises him to kill the prisoner or lose the kingdom, but Grimoaldo’s conscience prevents him from taking this action: he is caught in a web of conflicting feelings—fear, suspicion, love, and remorse.
Bertarido is reassured when a weapon is dropped through the bars of his prison cell. In the darkness he strikes out at what he believes to be an assassin—but it is Unulfo, come to help him. Even though he is wounded, Unulfo manages to get Bertarido to change out of the clothes he has been seen in. As the two men escape into the tunnel, Rodelinda and Eduige arrive—Rodelinda has insisted on rescuing Bertarido herself but finds only his clothes covered with Unulfo’s blood. She imagines the worst.
At the foot of Bertarido’s memorial Grimoaldo’s internal struggle continues. He ultimately acknowledges his cruelty and guilt. Exhausted, he falls asleep. Garibaldo attempts to assassinate Grimoaldo, but is stopped and killed by Bertarido, who gives himself up to Grimoaldo. Following Grimoaldo into the library, Bertarido dares him to condemn his own savior. Grimoaldo is himself ready to surrender and restores wife, child, and throne to the rightful king. His apology to Eduige goes unheeded at first, but eventually she forgives him. With reason restored, the survivors can envision and celebrate a happier future.