Set in New York at the turn-of-the-century, Tenderloin recounts the efforts of a crusading minister as he attempts to shut down the notorious red-light district on the West Side of Manhattan and the staunch defenders of debauchery out to stop him. The 'do-gooder' Reverend Brock is aided by a reporter, Tommy Howatt, with a hidden agenda of his own. Brock is unaware that Tommy, his supposed associate, is in league with a dishonest policeman, Lieutenant Schmidt. The two of them frame Brock with doctored photographs. In a twist, Tommy is converted to religion by a choir girl named Laura. As a result, in court, he admits the truth. A cleanup of the area begins. But Brock doesn't remain to witness the change - he moves his campaign on to Detroit.