At his death, the King of Lombardy left his domains in equal portions to his three children: Bertarido, Eduige, and Gundeberto. Also aspiring to these lands is Grimoaldo, Duke of Benevento, who is wooing Eduige. The avaricious Gundeberto has now been killed, and it is believed Bertarido also has died, leaving behind his wife, Rodelinda, as Queen of Milan.

In the royal palace, Rodelinda mourns for her husband with their son, Flavio. Grimoaldo has seized power, and she is now his prisoner. Grimoaldo offers the throne back to her if she will marry him. Despite her precarious situation, Rodelinda refuses him. Once pledged to Grimoaldo, the spurned Eduige vows vengeance on him.

Unbeknownst to anyone except his faithful aide Unulfo, Bertarido is alive and hiding nearby. Unseen, he watches Rodelinda weep as she and Flavio bring flowers to his memorial monument. Grimoaldo’s ruthless councilor, Garibaldo, arrives with another offer of marriage from Grimoaldo; if she refuses again, her son will be killed. When she finally accepts, the eavesdropping Bertarido is consumed with rage. Unulfo, his aide, restrains him from revealing himself.

Rodelinda announces her terms for marriage: She will only wed Grimoaldo if he personally kills Flavio before her eyes. Grimoaldo recoils at this, just as Rodelinda suspected he would. Garibaldo urges the wavering Grimoaldo to kill the boy, but Grimoaldo refuses. Unulfo rushes to Bertarido to reassure him of his wife’s fidelity.

Bertarido’s sister, Eduige, discovers her brother’s hiding place and promises to help him win back his wife and son. Acting as intermediary, Unulfo tells Rodelinda that Bertarido is alive and brings them together. Grimoaldo comes upon the happily embracing couple and threatens to imprison Bertarido. Rodelinda and Bertarido sing anguished farewells to each other before he is led away.

Eduige and Unulfo plot Bertarido’s escape from prison. Garibaldo urges Grimoaldo to execute Bertarido. When Unulfo breaks into Bertarido’s prison cell, Bertarido, thinking he is one of the guards, stabs him. Not seriously wounded, Unulfo leads Bertarido to the secret escape route Eduige has prepared. Finding their way to the empty cell, Rodelinda and Flavio discover Unulfo’s blood on the floor and fear the worst. Rodelinda is overcome with despair.

Meanwhile, Grimoaldo, tormented by his conscience, seeks sleep in the palace gardens. Garibaldi discovers him there and, eager to usurp the throne for himself, is about to kill him. Bertarido leaps from his hiding place and strikes down Garibaldo. Chastened, Grimoaldo now reveals his change of heart and relinquishes the throne of Milan to Bertarido and Rodelinda. And he makes amends to Eduige by offering to wed her. All express their joy and hail a brighter future.