Elmer Gantry (Burt Lancaster) is a hard-drinking, fast-talking traveling salesman with a charismatic personality. While traveling, he's drawn to the road show of Sister Sharon Falconer (Jean Simmons) and is immediately attracted to the saintly revivalist. He soon cons his way into her good graces and joins the troupe as a fiery preacher. Gantry and Falconer develop what her manager calls a "good cop/bad cop" routine, with Elmer telling the audience members that they will burn in Hell for their sins and Sharon promising them salvation if they repent. With Elmer's support, the group makes its way out of exclusively provincial venues and into Zenith, Winnemac. Falconer eventually admits to Gantry that her real name is Katie Jones and that her origins are humbler than she publicly admits.
Gantry's on-stage antics draw the attention of big city reporter and nonbeliever Jim Lefferts (Arthur Kennedy). Lefferts is shown to be torn between his disgust for religious hucksterism and his genuine admiration for Gantry's charm and cunning. The two men begin a public feud which increases the notoriety of both.
The success of the Falconer-Gantry team is mired by Lulu Baines (Shirley Jones), Elmer's former girlfriend. Baines fell into disrepair and became a prostitute when her affair with Gantry ruined her standing in her minister father's eyes. Gantry, acting as a moralist, unwittingly invades a brothel which Lulu works at, with police and media in tow and sends the prostitutes out of town. Lulu frames Gantry out of revenge for this and out of jealousy for his relationship with Falconer.
Baines blackmails Gantry and asks Falconer to bring money in exchange for incriminating pictures. Falconer brings the money, but Baines refuses to accept; it is unclear why.
Though Baines first offers Lefferts the story of Gantry's sexual indiscretion, he refuses, certain of the story's untruth. Later, when an angry mob threatens Gantry at the tent revival following the publication of the incriminating photos, Lefferts fights in Gantry's defense.
Lulu joins the congregation at this tent revival to see Gantry's humiliation. However, as she watches the mob curse Gantry and cover him with eggs and other produce, she is emotionally shaken and flees the scene. She returns to the brothel, which is now in a dilapidated state from Gantry's publicity stunt. The photographer who helped frame Gantry is there to collect his money. When Baines cannot pay him, he beats her. Gantry comes to Baines's rescue in the midst of this beating. He disposes of the photographer and apologizes to Baines. She soon publicly confesses to having framed Gantry.
Elmer returns to Sharon the night her tabernacle opens and tells her that he wants them to live like a more normal couple. Sharon is unable to give up her soul-saving ventures, though, and insists that she and Elmer were brought together by God to do His work. Sharon tragically dies in a fire at her tabernacle, unable or unwilling to see past her own religious zeal when the place is engulfed in a fire. Deeply saddened by Sharon's death and having reached something of a moral awakening, Elmer decides to stop evangelizing, quoting from the Bible: "When I was a child, I understood as a child and spake as a child. When I became a man, I put away childish things." ()