In New York City in the 1870s, as the city prepares for the upcoming election, Boss Tweed, Vincent Price and his Tammany Hall political machine are working hard to re-elect their candidates, including Mayor Oakley, Hobart Cavanaugh, in order to continue exploiting the coffers of the city and state. The one voice opposing Tweed's organization is John Matthews, Dick Haymes, a young naïve reporter for The New York Times, who begins a quiet, principled push to expose the corruption simmering beneath the gilded surface.
When Irish immigrant Timothy Moore, Albert Sharpe and his singing daughter Rosie Moore, Deanna Durbin arrive in New York City hoping for a better life, they are set upon immediately by Rogan, Tom Powers, one of Tweed's men. The illiterate Timothy agrees to vote twenty-three times for the Tammany ticket, and is rewarded with $50 and an invitation to Tweed's victory party, an invitation that marks the first hollow notch in a long, corrupt chain.
At the party, Rosie inadvertently overhears Tweed's latest plan to embezzle the city's coffers through the unnecessary renovation of Central Park. Fearing that Rosie may know about his scheme, Tweed appoints the unknowing Timothy to the post of Park Superintendent, a move that shows how far the machine will go to secure loyalty and cover its tracks.
Sometime later, John meets Timothy, the new Park Superintendent. Unaware that John is a reporter, Timothy reveals that some of the park's zoo animals are actually being raised for Tweed's consumption. After John's story appears in the paper, Timothy is fired, but when Rosie appeals to an infatuated Tweed to give her father another chance, he agrees. Also smitten with Rosie, John offers Timothy a job with his newspaper. Soon after, John tries to convince Rosie of Tweed's dishonesty, but is unsuccessful. Later that night, Rosie almost discovers Tweed's true character when he makes numerous, lecherous advances toward her during dinner, but is interrupted by Timothy, who mistakenly believes that he was invited.
After Rosie arranges a meeting between John and Tweed, the political boss offers to sponsor John's proposed novel if he agrees to quit his job at The New York Times. John refuses the bribe. Later, John discovers Timothy attending grammar school classes; with the help of a schoolteacher named Miss Murch, Mary Field the old man learns of Boss Tweed's corruption. When Timothy tries to tell his daughter about Tweed's true character, she refuses to listen, having become romantically involved with the married man, adding a personal stake to the political drama.
Through Tweed's influence, Rosie soon auditions for an opera company, and though she is offered a role in an upcoming production, Tweed insists that she be cast in the current show, showcasing how personal ambition and political power intertwine in this tense cityscape. Meanwhile, Timothy, upset over his daughter's involvement with Tweed, approaches John and offers to help him gain evidence against the political boss by breaking into city hall and examining the city's financial records. The two men are discovered by a drunken Mayor Oakley when he wanders into his office, but they trick him into giving his copies of Tweed's financial dealings to the newspaperman, a pivotal moment that fuels the investigation.
After their corruption is exposed in the newspapers, Boss Tweed and his associates prepare to flee the country, but Tweed offers no apologies to Rosie for his actions, stating his belief in the rights of the strong over the weak. After he leaves her, Rosie wanders through Central Park, where she is discovered by Timothy and John. After requesting her father's forgiveness, Rosie is reunited with John, and the film closes on a note that blends moral reckoning with a glimmer of personal reconciliation.
