 George Hollister (Jack Holt), crack investigator for the NYC police, is assigned to run down the enemy aliens who dynamited a Detroit munitions plant. The trail takes him to a customs inspector who is known to have honored fake passports provided by Leon Fenten (C. Henry Gordon), chief henchman of Drexel Stuyvesant (Clay Clement) who heads the sabotage ring. Aided by his partner Ray Nolan (Noah Beery Jr). Hollister arrests an agent named Reed (Robert Fiske) and takes his place using the fake passport. As Reed, he is met by Karol Roy (Cecilia Callejo), an innocent assistant, who takes him to Fenten. Working undercover, Hollister is still unable to learn the name of the head man, not even from dumb gang-member Hank Kircher (Maxie Rosenbloom). Stuyvesant warns Fenten of his suspicions about "Reed" and the latter gives Hollister an assignment as a test.