At the Hotel Compton in Gary, Indiana, postal inspector Harry Gruber is murdered by two men, Jack Webb as Joe Regas and Harry Morgan as George Soderquist. They dump the body in La Porte during a rainstorm, but just then a nun, Sister Augustine, Phyllis Calvert passes near them. Soderquist tries to distract her by helping unfurl her umbrella and pretending Gruber is drunk, but she reports the incident.
Postal inspector Al Goddard Alan Ladd is assigned to the case. He traces Sister Augustine to a convent in Fort Wayne, and recites to her a quotation from Martin Luther about personal responsibility. She then agrees to go to the police station, where she identifies Soderquist from a mug book, and then to Gary in the hope of identifying him there in person. She will stay at a convent there until she is needed to testify in court.
Soderquist is seen by Goddard and Sister Augustine with another gang member, Paul Ferrar, Stacy Harris as Paul Ferrar, but gets away. Meanwhile, Regas, whom Sister Augustine does not recognize, sees her and telephones Earl Boettiger, the head of the gang and owner of the Hotel Compton, Paul Stewart warning him to hide Soderquist. Later, when Soderquist rejects an order to leave town, they kill him.
Goddard realizes that as Soderquist has not left town, the gang must still be planning something. He talks to the Gary postmaster, who suggests a likely target for theft: a money shipment that arrives regularly in Gary by train but must change to another train, involving a seven-minute truck ride between stations. Goddard had asked him about the truck's drivers, one of whom recently declined a chance at a higher-paying job; it is Ferrar, and Goddard recognizes him from before.
Goddard, posing as a corrupt inspector, pressures Ferrar to meet the head of the gang. He then asks Boettiger to join the gang, promising to be a more reliable inside man than Ferrar. The plan is indeed to steal the money shipment, expected to be worth $1 million. Meanwhile, Regas is still worried about Sister Augustine and tries unsuccessfully to stage a fatal accident for her.
When Boettiger makes a last-minute change to the plan, Goddard has no choice but to phone the police from the gang's hotel suite. He is heard by Boettiger's mistress, hotel employee Dodie, Jan Sterling as Dodie, but she protects him because she does not want to be an accessory to his murder. She reports the planned crime to him in order to stay on the right side of the law, and says she is leaving town at once.
The robbery does not go as smoothly as hoped, because Regas neglects his part in the preparations to take Sister Augustine prisoner. When the gang meets, she blurts Goddard's name, revealing his deception. He pleads for them not to kill her, then buys time by starting a fight. As police arrive, a shootout ensues in an industrial district, ending in the death of all the gang members.
