Except for the switcheroo, it's a routine Falcon entry. The series was never big on mystery or atmosphere, staples of better whodunits like the Chan series. Instead, the entries lean heavily on sprightly charm supplied first by Sanders and now by Conway, and a steady stream of statuesque, well-upholstered girls, which this entry supplies in spades. Looks like RKO may have used the series to photograph some of their younger talent.
It turns out here after a couple false starts that the brothers are on the trail of Nazi spies, not surprising for 1942. However, I am surprised that the two actors generally deliver such engaged performances over the course of the entire series. After all, the Falcon role is basically one-dimensional, requiring only that the actor be suavely charming and not much else. I expect Sanders had become bored and quit before losing interest; at the same time, Conway picks up nicely on what has become a family trust.
The best scenes are with Conway and the sprightly reporter Randolph, whose well-scripted chemistry works. It's also a good chance to ogle high fashion, circa 1942. On the other hand, is the witless comedy relief from Clark, Gargan, and Barclay, blowing little lame holes through the proceedings. Anyway, the movie may be unique in film series annals for the way it creates the switcheroo, which really does amount to a startling departure. Otherwise, it's strictly a routine affair.