I read this again recently, although I first read it as a child - and still found it very familiar indeed.
The Five are on a cycling tour through the countryside, and everything is very pleasant and idyllic, with enjoyable meals by the wayside and swims in pools, until the Five meet Richard Kent, who accompanies them for a short while, but unfortunately leads them into considerable trouble, partly through his own lies and deceit.
Richard's wealthy father had had a former bodyguard known as Rooky, who had been dismissed as a result of Richard telling tales to his father, and as a result Rooky is thirsting for revenge. Unfortunately he catches sight of Richard after he parts from the Five, and Richard comes blundering into the forest where the Five are about to set up camp, seeking their help, and two of Rooky's henchmen come upon Dick and kidnap him, mistaking him for Richard, whom they do not know by sight.
Exciting adventures follow as a result of the others' late-night attempt to trace Dick and rescue him (with Richard tagging fearfully along); this leads them, via a moonlight bike ride and a wayside adventure, to Owl's Dene, a lonely, walled mansion which, as its secrets slowly unfold, turns out to be a veritable nest of assorted rogues. All the children are held there, and there seems no hope of escape until Julian thinks up a plan which will only work if Richard is able finally to prove his bravery.
This is one of the more exciting of the Famous Five stories. The plot thickens during the second half of the novel, and when the children are eventually able to send word to the police about what is going on, and the police are closing in, the criminals are getting into a siege mentality, and the danger for the children increases, especially from Rooky, who has seen all his neat plans come unstuck because of the children's actions, and whose desire for revenge extends to all of them now, not just Richard.
There are a couple of plot flaws. Given Richard's cowardice (which is something of a theme followed by the book, culminating in his big opportunity to redeem himself), as well as his entirely reasonable fear of Rooky, it is puzzling why he followed the others into the den of thieves simply because he dreaded being left behind all alone even more. Also, it is difficult to see why the children, especially towards the end, when they are all together, and getting more desperate, don't even consider trying to escape by climbing over the iron gates to the mansion. (They are outdoors some of the time within the grounds, more or less unsupervised, so the opportunity was there.)
Nevertheless, after the peaceful, holiday atmosphere of the first few chapters, you will not be able to put this down until you've finished. I have read adult books whose climax was less tense and intricate than this book.
The characters may not be portrayed in any depth or subtlety, but they are quite well differentiated: the children, and the criminals they get tangled up with, too - at least the ones who appear more than occasionally. For instance, Mr. Perton is cool and rational, always evaluating the best thing to do, whereas Rooky's desire for revenge causes him to act rashly (even from his own point of view), and in fact contributes to his and the others' final undoing.
The writing style is quite plain but very clear in Blyton's usual manner, and never gets in the way between the story and the reader. There are occasional touches of atmosphere which have, for instance, caused the moonlight adventure about half-way through to remain as a clear image in my memory ever since my childhood. The sense of atmosphere which sometimes appears in these books is probably helped by Eileen Soper's excellent original illustrations, brought back for the centenary Famous Five edition.
A thrilling children's book, and perhaps even an enjoyable read for older people who want something relaxing or nostalgic to read in an odd few hours.