The book gets off to a very slow start with far too much exposition.
Once we get the boys through the initial chapter, the action picks up quite a bit, providing plenty of thrillingly unreaslistic plot twists and nail biting escapes.
The book is largely a "Nancy Drew" style adventure, but with an interesting twist. It is set in Britain at the outset of WW1, and has many striking details of that time which must have been drawn from first hand observation.
Durston is keenly sensitive to war's moral ironies, and often places his rather nuanced adult views in the thoughts of his young heroes. Although this is unrealistic, it lends the book a kind of gravity that a typical yarn of youthful pluck and courage lacks.