Florence Fallon, the daughter of an elderly clergyman, is outraged when the church elders replace her father with a younger preacher after twenty years of ministry. She accuses the congregation of ingratitude and hypocrisy, and as the crowd departs, her bitter, impassioned speech catches the eye of Bob Hornsby, a promoter who sees an opportunity to profit from her faith.
Florence, billed as "Sister Fallon," becomes a traveling evangelist for the Temple of Happiness, with sermons broadcast on the radio. John Carson, a blind aviator veteran who contemplates suicide, is drawn to Florence's message and attends the crusade with his caretaker, Mrs. Higgins. At the crusade, Florence preaches from inside a cage of lions and challenges a congregant to join her inside it as a test of faith; John accepts the dare. Afterward, Hornsby shames the shills for not answering her calls during the sermon, while Florence begins to see how far her newfound platform could take her.
As Florence prepares to leave for Hornsby's party, John waits outside and brings her to his apartment, where he treats her to music and his ventriloquist dummy. Before long, Florence hires John to write religious hymns for her mission, cementing a tentative professional and personal bond.
When Florence arrives at Hornsby's party, their manager, Bill Welford, threatens to reveal their scheme unless he receives more money. After Florence finishes a sermon, Hornsby sensationally shows a newspaper article reporting Welford's death by an apparent suicide and professes his love for Florence, forcibly kissing her in a moment of coercive romance she rejects. Before he leaves, Hornsby takes Florence's house key, giving him leverage over her.
Florence returns to John’s apartment for a quiet, romantic evening, and the two share a kiss. Later, at Florence's house, Hornsby sneaks in and shows her a newspaper filled with planted stories claiming that Florence is ending her crusade for health and will be traveling to Palestine—actually a trip to Monte Carlo. Florence demands a retraction, but Hornsby blackmails her, threatening to expose her for embezzlement and to pin Welford's suicide on her.
Before Florence can act, she confides the truth to John, who responds that he still loves her. On the eve of her farewell sermon, John tries to convince her that his sight has been restored, but the truth still weighs on her. As Florence prepares to address the congregation, Hornsby punches John in jealousy. When Florence reveals the truth to the crowd, Hornsby orders a stagehand to switch off the lights. The plan backfires, a fire erupts, and the congregation flees in panic. Yet Florence refuses to abandon them and leads the singing as the blaze rages, while John is revived and manages to rescue her from the flames.
Six months pass, and Hornsby discovers Florence has begun working with the Salvation Army. A telegram from John arrives, suggesting his own eyesight could be restored. With renewed faith, Florence marches forward, while Hornsby watches with contempt.