George Hollister, 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, chief henchman of Drexel Stuyvesant who heads the sabotage ring. Aided by his partner Ray Nolan. Hollister arrests an agent named Reed and takes his place using the fake passport. As Reed, he is met by Karol Roy, 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. Stuyvesant warns Fenten of his suspicions about "Reed" and the latter gives Hollister an assignment as a test.