Treasury agent Frank Warren attempts to contact Manny Zanger, a man with access to information that would prove that mobster "The Big Fellow," the head of a major crime syndicate, is guilty of tax evasion, but Zanger is killed before the meeting takes place. Although the police capture the killer, they are unable to hold him because none of the witnesses will testify against him. The Treasury Department hopes to apprehend The Big Fellow on tax evasion charges, and to this end, confiscates bookkeeping records from his low-level associates. The T-men then arrest all the bookkeepers who work for the syndicate in order to compare their signatures to those on certain bank deposit cards. Before the T-men can complete this effort, however, Edward O'Rourke, the syndicate lawyer, obtains the bookkeepers' release. In frustration, Inspector Herzog, a police captain, quits the force. Sergeant Shannon, a policeman who, years earlier, abandoned his attempt to fight The Big Fellow, then shows Frank the record of Salvatore Rocco, a bookkeeper for the mob, whom he arrested before he was demoted to a desk job. When Frank learns that Rocco lives in the same neighborhood as Zanger's contact, he visits the apartment. Rocco's wife, angry because he has left her for another woman, admits that Rocco knew Zanger, but adds that she does not know Rocco's current whereabouts. She shows Frank a letter from Rocco, and Frank is pleased to discover that Rocco's handwriting matches that on the bank deposit cards. Frank and his associate persuade Rocco's girlfriend, Gladys LaVerne, to talk to Rocco. Gladys tells Frank that Rocco will testify if he gets federal protection and the reward. In exchange, Rocco asks his young daughter Rosa to bring him a notebook he had hidden at his former apartment, which contains the records of deposits he made for the mob. Before she can deliver it, however, Rocco is killed by the mob. When Frank returns from Rocco's funeral, his room has been searched and two waiting men beat him severely. Later, O'Rourke tries to make a deal with The Big Fellow and subtly threatens Judy, Frank's wife, who is staying at her parent's nearby farm. Deeply disturbed, Frank takes the next train to visit Judy and tells her that he intends to quit his job. After Frank's return to the city, Rosa and her grandmother visit Frank. Rosa's grandmother tells Frank that her husband died defying the Mafia in Italy, then gives him Rocco's book. A contrite Frank agrees to stay and fight The Big Fellow. The book contains almost all the evidence the Treasury Department needs to prosecute The Big Fellow. They then track down Sidney Gordon, another mob bookkeeper, in Los Angeles, and arrest him and his wife Muriel. Gordon agrees to cooperate and his testimony leads to the indictment of The Big Fellow and his associates. O'Rourke then sets out to buy off the grand jury. After O'Rourke is subpoenaed, he meets secretly with Frank and offers him a complete account of The Big Fellow's financial arrangements and also reveals that the jury has been bought. The mobsters discover his betrayal and kill him. Frank then uses O'Rourke's information to substitute a new jury for the corrupt one and The Big Fellow is sentenced to twenty years in prison.