This is yet another experiment in the all important detective genre before the genre settled into the few riverbeds we work with today. The experiment failed, which is why you don't find this celebrated. Its actually a very bad entertainment. Very bad indeed and after this Nick Carter would end.
Its only interesting if you study how the notion of film detection and noir evolved, and what branches died out... or if you are interested in how national identity is defined in film (or reflected if you are a gnostic).
This one tries to punch up the franchise with pretty girls, six of them who are apparently prostitutes though the relationship is so softpeddled, they are mentioned as "dancers." They are protected by a dumb blond who is so dumb it defies even movie logic.
One of these is a German girl who is reluctantly recruited into a German conspiracy against the US, a "fifth column." When she is condemned in front of members of the "cell," one decent man gets up to protest that he joined to make the world better. He is immediately beset by thugs and beaten to death. There is mention of concentration camps. The US hadn't yet entered the war, but it was ready.
There's a mystery of sorts here, how someone is stabbed in a sealed cabin on an airplane. But it is so contrived, so needlessly elaborate its funny, like the genteel whores that subliminally stand for American values.
Ted's Evaluation -- 1 of 3: You can find something better to do with this part of your life.