The argument
Prior to the opening of the opera, Grimoaldo has defeated Bertarido, King of Lombardy, in battle and has usurped the throne of Milan. Bertarido has fled, leaving his wife Rodelinda and his son Flavio prisoners of the usurper in the royal palace. Failing to secure support to recover his crown, Bertarido has caused it to be reported that he has died in exile, a ruse to be used in an attempt to rescue his wife and son.

Act 1

Rodelinda's apartments
Alone in the palace, Rodelinda mourns the loss of her husband Bertarido. The usurper Grimoaldo enters, declaring a long-hidden passion for her. He proposes marriage and offers her back the throne that is rightfully hers. She angrily rejects him (Aria:"L'empio rigor del fato"). Eduige arrives at the apartment looking for Grimoaldo. Grimoaldo, having previously been betrothed to Bertarido's sister Eduige, now tells her that as she once spurned him, he shall spurn her. After Grimoaldo leaves, the scheming Garibaldo, his counsellor, professes love for Eduige. She promises to return his love once she has had revenge on Grimoaldo (Aria:"Lo farò, diro: spietato"). Alone, Garibaldo reveals that his love for Eduige is feigned, and is part of a plan to gain the throne for himself (Aria: "Di cupido impiego i vanni").

A cypress-grove
Bertarido, in hiding nearby, reads the inscription on his own memorial and longs for his beloved wife Rodelinda (Aria:"Dove sei, amato bene?"). Along with his friend and counsellor Unulfo, he secretly watches as Rodelinda and Flavio, her son, arrive to lay flowers at his memorial. She weeps at her husband's fate. Garibaldo enters with an ultimatum for Rodelinda: either she agrees to marry Grimoaldo, or her son will be put to death. Rodelinda consents but also vows to demand Garibaldo's death when she returns to the throne. Bertarido, still watching, is aghast and takes Rodelinda's decision as an act of personal betrayal.

Act 2

A great hall
Garibaldo taunts Eduige, telling her that now, since she has lost Grimoaldo, she has missed her chance to become queen. Eduige satirically congratulates Rodelinda, noting her sudden decision to betray her husband's memory and marry his usurper. Rodelinda reminds Eduige of who is queen. Eduige vows vengeance on Grimoaldo. Eduige departs and Grimoaldo enters. Rodelinda sets out her terms for marrying Grimoaldo: he must kill Flavio with his own hands in front of her. Grimoaldo, horrified, refuses. After Rodelinda leaves, Garibaldo encourages Grimoaldo to carry out the murder and take Rodelinda as his wife, but Grimoaldo rejects the advice. He says that Rodelinda's act of courage and determination has made him love her all the more, though he has now lost hope of ever winning her. When the two advisors are alone, Unulfo asks Garibaldo how he could give a king such advice, and Garibaldo expounds his tyrannical perspective on the use of power (Aria: “Tirannia gliel diede il regno”).

A delightful prospect
Bertarido approaches the palace grounds in disguise, where his sister Eduige recognizes his voice. Unolfo brings word of Rodelinda's fidelity - also gratifying for Eduige - and Eduige agrees to help Bertarido rescue his wife and son. Unolfo promises to pass a message to Rodelinda that her husband is still alive. Bertarido rejoices at the prospect of reunion.

A gallery in Rodelinda's apartment
Rodelinda and Bertarido meet in secret, and are discovered in an embrace by Grimoaldo who fails to recognise her husband. Grimoaldo is outraged, believing that Rodelinda has taken a lover. To save her honour, Bertarido reveals his identity but Grimoaldo vows to kill him anyway, whoever he may be. The spouses bid each other a last farewell (Duet: "Io t'abbraccio").

Act 3

A gallery
Unulfo and Eduige make a plan to release Bertarido from prison: they will smuggle to him a sword and the key to a secret passage that runs under the palace. Garibaldo advises Grimoaldo to put the unknown man - whether Bertarido or not - to death. Grimoaldo is racked by jealousy, passion and fear.

A very dark prison
Languishing in prison, Bertarido receives the sword, the key and a written note. When Unulfo comes to release him, Bertarido mistakes the visitor in the darkness for the executioner and wounds him with the sword. Unulfo shrugs the injury off, and the two leave. Eduige guides Rodelinda into the cell. Finding it empty and with blood on the floor, they fear that Bertarido is dead.

A royal garden
Grimoaldo is tormented by remorse and flees to the palace garden, hoping to find a peaceful spot where he can seek solace in sleep (Aria:"Pastorello d'un povero armento"). Garibaldo, finding him unprotected, decides to kill him. Bertarido appears and kills the intended assassin; Grimoaldo, however, he spares (Aria:"Vivi, tiranno!"). Grimoaldo renounces his claim to the throne of Milan, and pledges himself once again to Eduige. He offers the throne back to Bertarido who accepts it once he is assured that his wife and son will be returned to him. There is general rejoicing.