New York City, 1944. Florence Foster Jenkins, portrayed by Meryl Streep, is an exuberant heiress whose passion for music knows no bounds. The story opens with a vibrant tableau at a club she founded, where Jenkins descends from above, embodying the muse of Stephen Foster, the composer of "Oh! Susanna," followed by her striking appearance as a fierce Valkyrie. Grateful for the support of her patrons, she returns home to her husband, St. Clair Bayfield (Hugh Grant), who sweetly recites poetry before she drifts into slumber. However, the scene quickly shifts as Bayfield tenderly removes Jenkins's wig, revealing her baldness and the frail nature of her health. He departs to a separate apartment he maintains with his mistress, Kathleen Weatherley (Rebecca Ferguson).
As Jenkins pursues her dream of singing, she engages John Totten for voice lessons, seeking a pianist to accompany her. Among many applicants, she is drawn to the timid Cosmé McMoon (Simon Helberg), whom she hires at an astonishing $150 weekly. Bayfield warns McMoon regarding Jenkins's peculiarities—cautioning him about the chairs of deceased celebrities, her strict avoidance of sharp objects, and the mysterious briefcase she carries—omitting the crucial detail of her lacking singing ability.
Upon beginning rehearsals, McMoon quickly grasps the staggering reality of Jenkins's "unique" talents, with her tone-deafness and erratic rhythm leaving him struggling to hold back laughter, much to the dismay of Totten and Bayfield. Florence is ecstatic when Arturo Toscanini (not an actor provided) visits her—she willingly parts with cash for his next performance, illustrating her unwitting status as a financial backer of New York's cultural scene.
As she decides to host a personal recital, both Bayfield and Totten express their reluctance to attend, leaving McMoon to tackle his anxiety over performing publicly alongside Jenkins. To preserve Jenkins's dignity, Bayfield fabricates a closed performance, inviting only acquaintances and easily influenced critics, which somewhat pacifies McMoon.
On recital night, while a few attenders smirk, the overall experience is victorious for Jenkins, who remains blissfully unaware of the mocking atmosphere. Afterward, at a rowdy party hosted by Bayfield, McMoon, inquisitive about the dynamics of Bayfield's relationships, is assured by Bayfield that Jenkins and he share a peculiar understanding.
Unexpectedly visiting McMoon's apartment, Jenkins discovers his untidy dishes and insists on cleaning as he plays piano for her. Through sentimental exchanges, they bond over music, recounting Jenkins's past struggles with mental and physical ailments due to an early marriage of constraint. A sudden fright emerges when she sees a knife, revealing the anxiety that plagues her.
Upon a subsequent visit, the doctor informs Bayfield about Jenkins’s deteriorating health, yet he never doubts her tenacity, even while grappling with the age-old affliction of syphilis. As Jenkins records more music artwork, public scrutiny escalates, prompting Bayfield to confront escalating chaos amidst yearning for his mistress, who ultimately departs when she feels sidelined.
When Jenkins decides to hold a monumental concert at Carnegie Hall, inviting soldiers to the event, both Bayfield and McMoon tremble at the thought of impending disgrace. Nevertheless, Jenkins, firm in her determination, presses on. As the auditorium fills, her psychological state teeters on the brink of panic, reinforced by her concern over McMoon’s tardiness—yet he reassures her they will shine together.
The performance oscillates between turmoil and triumph; laughter erupts, challenging Jenkins and McMoon's resolve. Amidst the disarray, a fervent Agnes Stark (not an actor provided) implores the audience to respect the performers' courage. Bayfield races to fend off a critic threatening to write a scathing review, and with the audience's mood shifting, Jenkins and McMoon push through, ultimately finding success.
Later, as they eagerly search for the day's newspaper to stifle any negativity, Florence is unprepared for the shock of finding a cruel review. This leads to a moment of madness as she almost succumbs to traffic, leaving Bayfield desperately pursuing her.
Confined to her bed in the aftermath, Jenkins questions Bayfield's loyalty and whether he was ever ridiculing her with the world. With care, he reassures her of his unwavering support, and she boldly insists that even if her singing is mocked, the fact remains that she has sung.
In closing notes, we learn that Bayfield opted for a modest lifestyle post-Jenkins while continuing to foster the arts with her wealth, and McMoon, although failing to gain acclaim in music, found new passion in bodybuilding.
