Der Rosenkavalier by Richard Strauss

The opera takes place in 1740s' Vienna, early in Maria Theresia's reign.

Act 1
Princess Marie Therese von Werdenberg, the Marschallin, has a secret and much younger lover, Octavian, who is with her in her bedroom when the opera begins. Octavian hides as the princess’s breakfast is brought in. Voices are heard and Octavian hides again, as the lovers fear it is the Marchallin’s husband returning.
The visitor is, in fact, her cousin, Baron Ochs zu Lerchenau. He has come to seek her advice in arranging his marriage to young Sophie von Faninal, daughter of a wealthy merchant who has recently been ennobled. It is customary for the marriage proposal to be presented, along with a silver rose, by an envoy, the Rosenkavalier (Knight of the Rose). The Marschallin suggests Octavian and shows the baron his portrait.
Octavian, meanwhile, has disguised himself as a chambermaid, “Mariandel”, and tries to leave the room. Ochs remarks on her resemblance to Octavian’s image and crudely flirts with her, jumping to the conclusion that she is the young man’s illegitimate sister. “Mariandel” manages to elude his advances and leaves.
The Marschallin’s levee proceeds. A tenor comes to serenade the princess and lawyers arrive to finalise Ochs’s marriage contract, with the baron arguing over the expected dowry. Two gossipmongers, Valzacchi and Annina, arrive to try and sell their latest scandalous tidbits. Ochs pumps them for information about “Mariandel” and engages them to find out more about her.
Observing herself in the mirror, the Marschallin realises she is no longer young. Alone, she laments her own early marriage and decides to end her relationship with Octavian before he inevitably leaves her. She sends him away, but becomes distressed when she realises she has not kissed him goodbye. Too late to call him back, she commands her servant to take the silver rose to Octavian.

Act 2
Sophie and her father are waiting for the Rosenkavalier, who will meet with her and her chaperone. She is concerned at the prospect of marrying an unknown man. Octavian arrives and presents her with the silver rose. Although he is acting as Baron Ochs’s envoy, it is clear the two young people are smitten with one another. Sophie’s father returns, with the baron and his rowdy companions. Ochs treats Sophie as if he already owns her and is overly familiar with Octavian, revealing he has an illegitimate sister. Ochs and von Faninal leave to discuss the marriage settlement.
Seeing how upset Sophie is, Octavian embraces her. They are seen by Valzacchi and Annina who report their discovery to Ochs. The baron is not concerned, seeing Octavian as no threat to him and his plans. Octavian challenges Ochs to a duel and wounds him. Sophie declares to her father she will not marry the marry. He responds by threatening to banish her to a convent and sends Octavian away.
Ochs is left alone with a consolatory drink, when Annina brings him a message from “Mariandel”.

Act 3
Annina and Valzacchi have teamed up with Octavian to entrap the baron. Ochs goes to the rendezvous with “Mariandel”, but his intended seduction is foiled by the actions of the conspirators. Annina arrives with a band of children, claiming to be Ochs’s wife. In the ensuing uproar, the police are called. Ochs claims “Mariandel” is his fiancée, Sophie, but when Sophie and her father are summoned, it is clear he is lying and they are outraged. Finally, the Marschallin appears, smoothing over matters with the police and persuading Ochs to relinquish his claim on Sophie.
Octavian is left with the two women. The Marschallin realises she is no longer central to his life. Sadly, she cedes her place to Sophie and leaves the two young lovers alone to embrace one another and their future together.