Across the river from Fort Benning, Georgia, the town of Phenix City, Alabama is under the thumb of a mob led by businessman Axel Monroe and his British cardsharp partner, Andy Collins. 'Sin City, USA' has become the town's nickname, attracting lowlife thugs from all over the South to serve as henchman and hit men for the mob. Anyone who tries to buck the mob is severely beaten or dumped in the Chattahoochee River face down. State and national politicians have been paid off to look the other way as illegal gambling, prostitution, loan sharking and other vices are practiced big time. Local officials are selected by the mob, with elections routinely stolen.
         Albert Patterson, a brusque, politically ambitious man in his mid-fifties, came to Phenix City with his family during the Depression because even then the town was booming. But after twenty years of seeing the mob gain absolute power, Patterson has finally had enough. When the home of a local reformer is blown sky-high, Patterson goes to see governor Larrabee Cousins to demand that the state take action. But Cousins declares the state cannot intervene in a local crime unless the attorney general says so. Knowing that he can expect no help from attorney general Frank Doyle, a close ally of Phenix City's corrupt district attorney Dick Moody, Patterson decides to run for state attorney general himself, to gain the law enforcement powers he needs to destroy the Phenix City mob.
        Albert's son John Patterson, recently returned home after army stints in both World War II and Korea, wants no part of his father's decision to run for office. He is ready to settle down with his pregnant wife and earn some money. John is well acquainted with the mob; he delivered groceries to the casinos as a kid. 'Mama' Rudene Ray, an ex-stripper who owns the most popular club in town, took a special liking to John in the old days and welcomes him home as if he were her own son. Mob boss Axel Monroe offers to set John up in his own practice -- a tempting deal for a broke ex-GI. Axel assumes, as does Rudene, that John is not interested in his father's incipient revolt since they have always appeared to be remote from each other. Albert Patterson is furious that John would even consider working with the mob. John points out that Albert himself has long accepted their patronage. Albert acknowledges as much but tells his son the situation has changed -- what was once a relatively benign operation has now become a cancer on the community.
         John's wife Katie pleads with John to stay out of his father's campaign because it's too dangerous. Axel Monroe assures John that Albert Patterson will not win the election, that his practice will be destroyed as a result of the campaign and that John more than ever needs to align himself with Monroe, But his father's attractive secretary, eighteen year-old Laura Hollis, has brought John under her spell. She persuades him to join the campaign in its final weeks. John is unaware that deputy sheriff Cecil Diggs, who owns a prostitution ring and has killed six people 'in the line of duty', has been tailing him and noting the time he has spent with Laura Hollis. Diggs tips off John's wife about what looks like an affair. She confronts John and tells him she is leaving him.
         When Albert Patterson wins the election, he seeks to reconcile with his son, but before that can happen Albert Patterson is gunned down in the street. John is shocked and filled with rage. Some of the town's citizens begin to take up arms, vowing to taken on the mob themselves. But John demands they abide by the law. He will now seek vengeance on his own terms. That night John hears a car screech to a halt in front of his house -- he thinks the mob has come after him. But instead he opens the door to find pugnacious, foul-mouthed National Guard general Walter "Crack" Hanna, sent by the governor to keep the lid on Phenix City in the wake of the murder. Hanna's only charge is his son Pete, acting as his driver. Hanna tells John he intends to stir up trouble and leak it to the newspapers until somebody confesses to the killing, or else the governor finds the courage to send in the national guard and take over the town lock stock and barrel -- in Hanna's view the only way the Phenix City mob can be defeated.
         Meantime deputy sheriff Cecil Diggs and district attorney Dick Moody frame John for his father's murder, claiming the two men were both in love with their secretary Laura Hollis. John gets out on bail thanks to Mama Rudene, who, like Axel Monroe, claims she has no idea who committed the murder. When the lone eyewitness to the crime ends up dead and the governor disappears on a fishing trip to avoid taking any action in the case, John, Crack Hanna and the latter's son Pete are on their own, threatened on all sides on the dark streets of Phenix City.