The Marschallin and her lover Octavian on the »morning after«: »How you were! How you are!« (»Wie du warst! Wie du bist! ...«). A commotion is heard outside – has the Marschallin’s husband returned unexpectedly? No – it is only Ochs, a country cousin, with two requests: a suitable »nuptial messenger« (Rosenkavalier) for his impending marriage to Sophie von Faninal, the daughter of a wealthy arms dealer raised to the nobility by the grace of Her Majesty, and a consultation – free of charge – with the Marschallin’s notary regarding the marriage contract. Octavian, disguised as a chambermaid, catches Ochs’s eye, who suggests a rendezvous. Once dressed, the Marschallin succumbs to melancholy and reflection on the fleeting nature of things: »For time is a mysterious thing« (Die Zeit, die ist ein sonderbar Ding). The Marschallin suspects that sooner or later her lover will abandon her for a younger woman. Octavian is asked to serve as the bridegroom’s messenger and present the silver rose at the home of the Faninals.
Sophie is awaiting her future husband (whom she has not yet met), when Octavian – looking like a fairy-tale prince – turns up to herald his arrival. The two find themselves captivated by each other. The boorish Baron Ochs auf Lerchenau doesn’t stand a chance against Octavian, and the bride refuses to go through with the wedding. Octavian steps in to protect her, and he and Ochs clash. The act closes with apparent good news for Ochs: »a certain Mariandel«, the Marschallin’s alleged chambermaid, has agreed to meet him for a rendezvous. The unwitting Ochs never suspects that Octavian is behind it all.
The terrace of an inn at the Prater. With the help of two fellow schemers, Valzacchi and Annina, preparations are underway for the undoing of Baron Ochs auf Lerchenau. Ochs arrives for his tête-à-tête with »Mariandel«, whom he fails to recognize as Octavian, and carefully planned chaos ensues. Wearing a disguise, Annina claims to be Ochs’s wife, whereupon Ochs – on the brink of losing his mind – calls for the police. They, however, interrogate him, and he is unable to prove his identity. All of the other characters turn up at the inn: because of the scandal Ochs has caused, Faninal annuls the marriage contract, and the Marschallin gets rid of the police commissioner, saving the Baron from more serious consequences. In the end, only the triangle remains: the Marschallin – Octavian – Sophie. The Marschallin renounces her claim to her lover. Octavian and Sophie marvel together: »It is a dream, it cannot really be true, that we two are together« (S’ist ein Traum, kann nicht wirklich sein, dass wir zwei beieinander sein).