Postal Inspector Al Goddard is assigned to investigate the mysterious murder of a fellow officer. The only witness to the crime is Sister Augustine, who identifies the photograph of Joe Regas as one of the assailants. This leads Goddard to a seedy hotel where he learns that Regas is a member of a gang headed by Earl Boettiger, and he soon discovers that the gang is planning a million dollar mail robbery. Posing as a corrupt Postal Inspector, Goddard threatens to frame one of the gunmen unless he is cut in on the deal. The gang is very wary of Goddard, but he plays his part shrewdly and wins their confidence. Then Dodie, hotel stenographer and girlfriend of Boettiger, discovers that Goddard is working with the law. But instead of telling Boettiger, she walks out. The big hold-up is pulled off successfully. Regas, who has been assigned to block the roads, kidnaps Sister Augustine and brings her to the shack where the gang meets to divide the loot. When Regas tries to shoot the nun, Goddard kills him. Sister Augustine, in her excitement, unwittingly reveals Goddard's true intentions. Boettiger and Goddard engage in a gun duel as police and Postal Officers close in. Members of the gang are cut down one by one, and just as Goddard's gun jams, Boettiger goes down before a burst of machine gun fire.