The story is pretty straightforward - Tintin gets recruited by a Balkan culture expert to travel to the mysterious land of Syldavia, where he crucially averts a plot to deprive the young king of his throne, engineered by an internal revolutionary movement which is a proxy for the neighbouring dictatorship of Borduria. There are lots of lovely Balkan/Slavic touches - although Syldavian spelling is closer to Polish than to the Balkans, the towns clearly have minarets and Cyrillic is used; the landscape and army/police uniforms are clearly drawn from the Balkan kingdoms between the wars. The small countries of south-eastern Europe are an easy target, but sometimes this can be done well.
But in fact the Balkans are mere protective coloration for what Hergé was really writing about. The unusually realistic depictions of the Warandepark and Avenue Louise in the early pages give it away. King Ottokar, running a small democracy in fear of annexation by its authoritarian neighbour through a front organisation, is not (as I have heard some speculate) Michael of Romania, but a slightly romanticised Leopold III of Belgium. The Bordurian plot to invade Syldavia could have been based on the Gleiwitz incident, were it not for the fact that it was published in Le Petit Vingtième in the summer of 1939, shortly before the Gleiwitz incident actually happened. Less than a year after Tintin En Syldavie had finished its original run, Belgium was occupied not by the sinister Bordurian activist Müsstler but by a bloke with a similar name.
And considering the general perception that Hergé was not exactly vigorous in resistance to Nazi occupation, it's a bit redemptive to see this story putting down a marker before it actually happened.
Also, given Tom McCarthy's speculation about Hergé's ancestry, it's amusing that he draws himself into two of the court scenes...
This was a good jumping-off point for my lifelong affection for Tintin, and I think I would still recommend it as a starting point today for people who for whatever reason have never yet tried it. The best of the pre-war albums is The Blue Lotus, but to really enjoy it you have to have read the inferior Cigars of the Pharaoh first. King Ottokar's Sceptre works well as a standalone adventure. (Even without Captain Haddock.)