Before passing away, Ariberto, the late king of Lombardy, has divided his reign between his two sons Bertarido and Gundeberto: the latter is supposed to rule in Pavia, the former in Milan. A dispute breaks out. Gundeberto calls for Grimoaldo, Duke of Benevent, to support him. In return he promises to give him his sister Eduige in marriage. Before Grimoaldo arrives, Bertarido kills his brother Gundeberto. He then flees from Grimoaldo's attack, leaving his wife Rodelinda and his son Flavio behind. From abroad he sends fake news of his death and starts to make his way back to Milan incognito.
Rodelinda laments her husband whom she believes to be dead. She turns down the courting of Grimoaldo. Grimoaldo seeks advice from his ally Garibaldo, Duke of Turin. Whereas Rodelinda wouldn't give in to his advances, Bertarido's sister Eduige is pressing him to marry her. Garibaldo suggests being firm towards both women. Upon Eduige's appearence, Grimoaldo accuses her that she didn't accept his proposal when he had not yet gained power. Now that he is king he doesn't want her any more. Eduige turns to Garibaldo who feigns being in love with her. He promises her to fight against Grimoaldo if she marries him instead. In reality, he wants to use her to claim the throne himself. When Bertarido arrives he finds himself confronted with a memorial erected in his honour. Unulfo, officially a counsellor to Grimoaldo but secretly loyal to Bertarido, urges him to hide. Bertarido observes his wife together with their son Flavio laying a wreath at the memorial. Suddenly Garibaldo turns up and threatens to take Flavio in hostage if rodelinda doesn't agree to marry Grimoaldo. Rodelinda declares being ready to do so; but the first thing she will demand from Grimoaldo as a wedding gift will be Garibaldo's life. Grimoaldo reassures Garibaldo promising he will not sacrifice him once he will have conquered Rodelinda. Bertarido now believes his wife to be unfaithful to him. When unulfo suggests telling Rodelinda that her husband is still alive, Bertarido forbids him to do so. He wants to see if she is really going to marry his enemy.
Eduige is still in love with Grimoaldo. Full of hate, she confronts Rodelinda, who is apparently willing to marry him. When everyone is assembled for the wedding, Rodelinda comes up with a condition Grimoaldo has to comply with: she will not become his wife unless he kills her son Flavio before her eyes. Grimoaldo is desperate; he doesn't feel to be up to such a crime. Garibaldo is convinced that a tyrant can only stay in power if he is ready to be cruel. Unulfo is horrified, but tries not to lose hope.
Eduige comes across Bertarido and recognises her brother. He tells her that he no longer wants to be king of Lombardy, but only save his wife and son. Unulfo arrives bringing the news that Rodelinda has turned down Grimoaldo again. Bertrido decides to meet his wife. Unulfo tlls Rodelinda that her husband is alive. When the two embrace, Grimoaldo burst in. Bertarido wantint to save Rodelinda's honour drops his disguise and reveals his identity. Grimoaldo doesn't believe him but hash mi arrested anyway. The two must say farewell to each other.
Eduige promises to help Unulfo and gives him the keys to Bertarido's prison. She will also provide Bertarido with a weapon. Bertarido is lamenting his fate when suddenly a knife is thrown into his prison through an opening in the ceiling. He succeeds in cutting his bonds. When Unulfo opens the door of the prison Bertarido fears his henchman has come and attacks him with the knife. Despite getting wounded, Unulfo shows him the way out. They leave Bertarido's blood-stained clothes behind. When Rodelinda enteres the prison together with Eduige and Flavio she sees the blood on the clothes and believes once again that her husband is dead. Garibaldo finds Grimoaldo sleeping. When he prepares to kill him, he is surprised by bertarido saving Grimoaldo's life by overwhelming Garibaldo and killing him instead. Rodelinda arrives; she can't believe her eyes: her husband has escaped once more from death. Grimoaldo is moved by Bertarido's noble deed. He turns back to Eduige, claiming that he will marry her and become the ruler of Pavia. Bertarido shall regain his throne in Milán and be the king of the Lombards. Rodelinda, Bertarido and Flavio are finally reunited.