Hungarian prima donna Nella Vago sings Tosca in Venice and, while her performance is politely received, her music teacher Rudig tells her that she will never be successful until her singing is more passionate.  Although Nella is infuriated by Rudig's demand that she must sing with her soul as well as with her voice, she is distracted by a mysterious man repeatedly walking in the courtyard below her balcony.  Rudig intimates that the man is the gigolo of the Marchesa Bianca San Giovanni, a former diva with a notorious past.

Later that night, Nella is kept awake by the passionate whispers of a honeymooning couple staying in the room next door, and she decides to return to Budapest that night.  Accompanied by her butler Conrad, her maid Emma, Rudig and her stuffy fiancée, Count Albert von Gronac, Nella boards the train bound for home.  Her anonymous admirer is also on the train with the Marchesa, and Nella burns with jealousy.  Rudig again tells Nella that she needs to experience love if she is ever to become a great singer, and she gently turns him down when he offers himself as a suitor.

The next day, Rudig tells Nella that Fletcher, an agent for the prestigious Metropolitan Opera Company in New York, is signing European artists.  Nella also discovers that afternoon that Albert is having an affair with another diva.  Furious at his betrayal and her own colorless existence, Nella goes to the hotel where her admirer, who is named Jim, is staying.  Using a flimsy excuse to get into his room, Nella vacillates between fulfilling her desires or fleeing.  Jim realizes that she believes him to be a gigolo, but nonetheless confesses his love and challenges her to stay with him "tonight or never" and experience true passion.  Nella gives in, but leaves the next morning before Jim awakens.

That night, Nella sings Tosca again, and her emotional performance is acclaimed by the audience and by Rudig.  Nella returns home from the opera house and finds a Metropolitan contract sent to her by Fletcher.  She is delighted by the contract, but overcome with guilt over her actions of the night before.  Jim arrives to return the emerald necklace she left in his hotel room as payment for his "services," and she tells him that she never wants to see him again.

Later, however, she goes to him and asks him to give up his shameful lifestyle for her.  When he in turn asks if she would give up her career for him, she gladly tears up the Metropolitan contract.  Just then, the <archesa enters and reveals that she really is Jim's aunt and that he is Fletcher of the Metropolitan.  Nella is overjoyed, for now she can have love and a successful career.