[In the following review, Barnacle describes Spies as fascinating, pleasurable, and powerful, despite its implausibility and recycling of familiar themes.]

Books read in adulthood almost never seize and enwrap your imagination like the books you read as a child, but here is one that might do the trick. [Spies] is about children and the intense, “half-understood” world that they inhabit, and it has the brevity and compactness of books written for the young, yet neither of these factors can quite explain its remarkable grip. It recycles some familiar themes and suffers from a major drawback in the area of plausibility; all the same, it works like a charm.

Stephen, an old man living somewhere abroad, is assailed one summer by the overpowering reek of a privet hedge, which brings on a Proustian recollection of his wartime childhood in an outer-London suburb. Back in 1943 or thereabouts, his best friend is Keith, who lives in the same cul-de-sac. Keith, a bossy only child, is slightly but significantly posher than Stephen, and sets the tone for their fantasies. The as-yet-unbuilt cableway for sending messages between their houses: Keith's idea. The string of undetected murders committed by Mr Gort down the road: Keith's idea. The secret society operating from “Trewinnick, the mysterious house where the blackout curtains are always drawn”: Keith's idea.

Keith's parents, uniquely in the close, have a car. It's up on blocks, “to prevent its being commandeered, as Keith explained, by invading Germans”. Stephen foolishly asks “if the Germans, with the evil ingenuity for which they were notorious, might not take the wheels from the wall and put them back on the car”. Keith, for once not narrowing his eyes with menace, says that his father has locked the wheelnuts in a secret drawer.

Out of the blue, Keith produces a new fantasy that is distinctly odd. “My mother,” he announces, “is a German spy.” Not, you notice, his ghastly father, all sinister smiles and “old bean”, much given to caning Keith for trivial misdemeanours, but his very nice and rather glamorous mother. It soon becomes clear which parent Keith himself takes after.

Reading the mother's diary on the sly, the boys discover a page marked with an X each month, coinciding with the dark of the moon. That'll be when her controller parachutes in to debrief her, then. So far, so ridiculous; but when they start following her, they find that she really is up to something covert. Out by the railway embankment, she has a dead-letter drop: an old Gamages Croquet Set box hidden in the grass, containing, on inspection, a packet of 20 Craven A and a sheet of notepaper marked with an X.

Whatever is going on (and the truth is more complex and involved than you might think), it is probably nothing to do with German spies and certainly nothing that will be improved by the meddling of small boys. In fact, the reader grasps the who, what and wherefore early on, about halfway through, but the story loses none of its fascination. A degree of foreknowledge only makes you more apprehensive.

The major drawback is the way the boys fail to distinguish between make-believe and reality, or rather the way they give them equal weight. Michael Frayn, in the guise of Stephen, explains all this with clinical precision. In their treasure chest, an old trunk, the boys keep a carving knife, salvaged from a bomb-site, which “both is and is not” Keith's father's bayonet, the one that killed five Germans in the Great War. “In its physical nature”, it may be only a carving knife. “In its inward nature, though, it possesses the identity of the bayonet.”

Fair enough. But when Stephen extends the logic to claim that he both believes and does not believe that Keith's mother is a German spy, one feels a certain strain. Surely even a ten-year-old knows better than that. A girl, with all the precocious worldly-wisdom of girls, offers a better theory for the lady's furtive behaviour, and Stephen immediately discards the spy fantasy, only to resume it straight afterwards. One can see the double-vision effect Frayn is trying for, but it doesn't quite come off.

What carries the day is the sense of adult drama off-stage, the carefully chosen period detail—pigbins for household scraps on street corners, Double BST prolonging daylight till after 10 pm—the scrupulous style and the powerful, tense scene-setting. Familiar elements from The Go-Between and What Maisie Knew emerge, but are quickly forgotten in the sheer, immersive pleasure of reading.