This is an interesting story of Blyton's because is was written during the opening stages of World War II, set during World War II and involved the antagonists of World War II. Actually, I believe that it is the only book of Enid Blyton's that actually makes reference to the war. It is interesting how Blyton portrayed the Germans, that is as 'the enemy' and the 'crooked cross' but she never actually mentions them by name. Maybe it is because, from the eyes of a child, it was something that was still far off, though to many in England at the time, especially during the blitz, the reality of war had struck home.
In this story four children go out on a sailing trip to an island but end up getting caught in a storm and sent off course. At first they are simply trying to find out how to survive on a deserted island in the middle of the North Sea however during their exploration they come across a cave full of food. They then see a sea plane (adorned with the crooked cross) land and then take off, and after a little more exploration, and a hazardous swim, they stumble upon a submarine base.
This story is a little different that many of Blyton's other books as it jumps straight into the action, and while it is not as violent, or as bleak, as the sequel, it does seem to come off a little different than her later adventure stories such as the Famous Five. In a way, in her later books, the children all seem to be somewhat protected, despite coming up against some rather unsavoury characters, but maybe it is because they are never really cut off from the rest of the world as they seem to be in these stories.
Unlike the Famous Five, where there always seems to be a farm house, or at least a friendly adult, nearby, or the Secret Seven, which occurs in the children's home town, the Adventurous Four, in both stories that I have read, are completely cut off from the rest of the world, and in many cases everything seems to be touch and go. Also, the risk of serious injury, or even death, seems to be much higher. In fact, at one point, there is a threat that the children would be taken back to Germany and placed in a POW camp (or even a concentration camp).
I thought this one was better than the sequel (surprise, surprise) but it still does not seem to have the child like quality that some of her other books have. However, I do feel that maybe the target audience is somewhat older than the Famous Five, though maybe not as old as, say, a full blown teenager (which didn't exist at the writing of this book, by the way).