If you simply take the movie at face value, it's an exciting story from the silent era. This came out during that nervous time between the World Wars. People remembered Russia's recent actions in WWI, and remembered Japan's war with Russia, a few decades past, as a clear indication of a force affecting the West. Weimar Germany represented another unstable force. Soviet infighting was also recent news, as the Trotsky-esque bad guy reminds us. Fiction about international intrigue had plenty of fact to work with - so Lang produced this remarkable work. The modern music suits the movie beautifully. Although other instruments appear, solo piano carries most of the musical narration. Even though it's not synchronized to the imagery on screen, onomatopoetic passage trill to a ringing phone, syncopate to the staccato of Morse Code, and hammer out gunshots, when not simply voicing the general mood of the scene.
Amid the excitement that must have been high-budget in its time, we see the origins of the modern spy-movie staples we see today: elaborate and fallible plots on the good guy's life, the bad guy's lair coming down around his ears in the end, chases, a babe who's not just there to be saved, and a little moral ambiguity. Sonja wasn't 100% on the good guys' side, at least to start, even if she came around in the end.
Anyone with the Bourne movies or Mission Impossible in mind will find the pacing sleepy at best, the effects ineffectual, and the acting as stylized as Kabuki theater. Today's movies learned from this one, though, and from the eighty more years of development between then and now. Taken by itself or as the progenitor of modern spy flicks, it remains an important and engrossing movie.