Charles Booth won an Oscar for his writing on this early drama-documentary depicting the hunt by the FBI for an established network of Nazi fifth columnists long since operating in the USA. It falls to agent "Bill Dietrich" (William Eythe) to infiltrate the cell and to find out who is ultimately giving the orders - the mysterious "Mr. Christopher". Reporting to "Insp, Briggs" (Lloyd Nolan) he treads a perilous path as his newfound friends doubt his backstory and suspect him of being a double-agent. I was put off by the overly earnest narrative from Reed Hadley, and the acting is all pretty lacklustre aside from Leo G. Carroll as the duplicitous "Col. Hammersohn" who is feeding the information to "Dietrich" whilst simultaneously trying to verify his identity. The ending is all too predictable and that really lets it down quite badly. For such a sophisticated network of spies to be quite so easy to identify is doubtless meant to be a testament to the skills of the wartime FBI, but as a device for a story, it lacks credibility: the fire escape, really? Henry Hathaway keeps it moving along well enough but the story leaves just too obvious a trail of breadcrumbs for it to be intriguing, or plausible.