In 1944, Florence Foster Jenkins is a New York City socialite heiress who founded the Verdi Club to celebrate her love of music. St.Clair Bayfield, a British Shakespearean actor, is her manager and long-time companion. Florence lives in a grand hotel suite, while Bayfield lives in an apartment with his mistress, Kathleen Weatherley. Florence suffers from long-term syphilis, contracted from her first husband.
Florence decides to resume singing lessons after she sees Lily Pons in concert with Bayfield. She hires pianist Cosmé McMoon, who is shocked by her terrible singing, but Bayfield and vocal coach Carlo Edwards, the assistant conductor at the Metropolitan Opera, pretend she is wonderful. Bayfield warns McMoon of dire consequences if he criticizes her.
Bayfield arranges a small recital, handpicking the attendees. Loyal Verdi Club members watch respectfully, but others barely contain their laughter. Encouraged by her good reviews, she arranges to make a recording at Melotone as a Christmas gift for members of the Verdi Club. Florence gives McMoon a copy and recalls that Bayfield was an unsuccessful actor and that she hid negative reviews from him. She also informs McMoon of her history as a piano player and teacher, having once played for the President as a child. McMoon realizes that Florence is not as musically inept as he had thought.
Florence and McMoon write and perform original songs together. One song is broadcast on the radio, to the horror of Bayfield and Kathleen, though many listeners find it comedic and enjoy it. Florence informs Bayfield that she has booked Carnegie Hall for a performance and will give away a thousand tickets to soldiers. Bayfield fails to dissuade her. He gets into a fight with a group of men laughing at Florence and McMoon's song at a bar. Kathleen, resenting the lack of attention, leaves him.
McMoon confides to Bayfield that he fears that the recital will humiliate him and ruin his career. Bayfield replies that he gave up his acting career to support his wife and urges McMoon do the same for his friend. With Bayfield playing emotional blackmail on McMoon's every heartstring, McMoon reluctantly agrees to accompany Florence, though fully expecting to flop spectacularly at Carnegie Hall.
The concert is packed and attended by celebrities such as Cole Porter and Tallulah Bankhead. McMoon arrives late which unnerves Florence, but Bayfield and McMoon encourage her to go on – Florence then writes McMoon into her will backstage before the two go out onto the stage. When Florence begins singing, the soldiers laugh and shout. Her supporters and friends, however, scold them and cheer for her to keep singing. She continues her performance. However, New York Post columnist Earl Wilson tells Bayfield he will write a damning review and refuses his bribes.
Bayfield, with McMoon's help, buys every copy of the New York Post in the neighborhood and throws them away. Despite their best efforts Florence hears about the review, retrieves a copy of the review from a trash can, and is so upset she collapses and becomes severely ill. As she is dying in bed, Florence imagines herself singing beautiful opera, and takes a bow with McMoon and Bayfield to a standing ovation. She tells Bayfield that though people may say she could not sing, no one can say she did not sing.
