Gerald I. Latimer, Lee Bowman, is an ambitious young attorney who teams up with Mayor Daniels, Samuel S. Hinds, and district attorney candidate Turnely to win elections on a pledge to crack down on the city’s criminal rackets; secretly, he forges an uneasy alliance with Matty, John Litel, one of the city’s most powerful gangsters, to pave his path to the U.S. Senate in exchange for future political favors.
Realizing the crackdown is real, Gerald warns Matty that Turnely and Daniels intend to go hard after crime, so Matty arranges Turnely’s murder. Gerald is appointed special prosecutor, and he meets the investigative team: Gordon McKay, Van Heflin, a forensic scientist, and his assistant, Jane Mitchell, Marsha Hunt. They examine the body and identify the killer, who dies while trying to evade capture.
Gerald stage-manages high-profile arrests of crime figures, but Eddie Wright, Eddie Quillan, a restaurateur harassed by racketeers, sees through the charade and realizes the crackdown only targets small operators. He decides to speak directly to the mayor about what must be done. He goes to the mayor’s house, his presence alarms the mayor’s wife, and the police are alerted, sending him home.
Meanwhile, the mayor questions Gerald about a large insurance policy he bought; when Gerald refuses to answer, the mayor declares his intention to investigate the source of Gerald’s funds. Wary of exposure, Gerald turns again to Matty, but Matty insists that Gerald deal with the mayor himself. Gerald plants a bomb in the mayor’s car, and the mayor dies when it explodes.
The police suspect Eddie; some circumstantial evidence points to him, but Gordon remains skeptical and continues the investigation, even as Gerald calls for Eddie’s arrest.
Gerald spends long hours in the crime lab and eventually falls in love with Jane. He asks for her hand in marriage, but she declines, explaining she cannot marry and quit her job until the double homicide case is solved. When Jane tells him that Gordon has concluded the killer would leave gunpowder under the nails, Gerald rushes to scrub his own hands, avoiding suspicion.
Later, Gordon finds a note in the mayor’s office implicating Gerald. He does not tell Jane, and instead secretly obtains a hair sample from him. After analyzing it, Gordon tells Jane he has found the killer but won’t reveal the name. When Jane and Gerald meet again and she agrees to marry, she reveals that Gordon has identified the killer through the hair, and Gerald realizes he must kill his friend.
Gerald arranges a meeting with Gordon and Matty, and gives his car keys to Jane so she can drive home. She notices a cigar cutter on the key ring and realizes it could have been used to cut bomb wires. She takes it to the crime lab for examination.
Gerald retrieves a gun from Matty and heads to the crime lab, where he confronts Gordon and demands the evidence incriminating him. Jane overhears the shouting from the lab, and Gerald confesses as Gordon confronts him. Gordon overpowers Gerald and takes the gun, and the police arrive shortly after, arresting both Gerald and Matty.
Gordon, now in love with Jane, proposes, and she accepts.
