The Five go cycling over to camp on Billycock Hill, close to the farm where a friend, Toby, lives. Also nearby is the butterfly farm owned by Mr. Gringle and Mr. Brent, where everything just doesn't seem quite right: the two butterfly experts are themselves strange, and one doesn't seem to recognize a Fritillary, and goes moth-hunting on a stormy night when no moths would be around; also, old Mrs. Janes who lives there seems constantly afraid of what her "bad" son will do if strangers visit the farm.
There are other strange events, too. For instance, the Billycock Caves are nearby, and the children go exploring, only to come bolting out in panic after hearing strange noises in distant caves.
Nearby is a military airfield which is thrown into disarray that same stormy night that Mr. Brent was out moth-hunting, when two top-secret experimental aircraft are stolen and flown overseas, one apparently piloted by Toby's cousin, Jeff, who seemed so decent and admirable to everyone, especially the doting Toby, that *surely* he couldn't be a traitor to his own country? But he and a colleague are the only airmen on the airfield who are missing, so the evidence seems pretty conclusive.
The Five and Toby investigate, thinking that maybe these disparate strange events are related somehow; they focus their attention on the butterfly farm, which seems to be at the centre of things, and a plot gradually emerges, and everything eventually falls into place, with a few shocks and surprises along the way.
While this story is quite engrossing, as are all the Famous Five books, and contains its share of surprises and revelations, it seems less exciting, and the plot a bit less focused, than many of the other Famous Five books. The children barely come into contact with their opposition, if you don't count the enigmatic butterfly men, who may or may not be real antagonists, and certainly not the main ones; the only contact with anyone more sinister than the butterfly men occurs in one brief burst, unlike most of the other Famous Five books, where the conflict between the two sides is usually far more direct, and much longer. This remoteness of the real antagonists probably accentuates the effect the story gives of being less exciting, less focused. The story itself seems rather episodic, not building up momentum in the same inexorable way that some of the other stories do. It might be a flaw that the most exciting moment just referred to comes several chapters before the end of the book. And it *is* only a moment, not several chapters long like many other Famous Five climaxes, and it is less exciting than most of those other climaxes even while it lasts.
Also, a significant flaw in the nuts and bolts of the plot is where the two stolen planes crash into the sea, killing the two pilots (extremely rare instances of death occurring in Enid Blyton's novels, albeit off-stage). Unless the planes were sabotaged (which was not mentioned, and wouldn't fit in with the plot), it just lacks credibility that *two* planes should crash (presumably accidentally) at the same time, and even the fact that it was a stormy night does not seem sufficient explanation, although you are left to presume that the storm did it, because no explanation was given for why the planes crashed. (I suppose the storm *could* do it, if it was extremely severe - but it didn't seem *that* severe.)
Those negative things said, the book is certainly as readable as any of the other Famous Five books, and you *do* keep reading to find out what happened - especially if you are indulging in a little nostalgia and reading it again for the first time in perhaps 35 years, as I did recently, and have forgotten most of the details of the plot.
