In a rigid, stratified future Britain, a young student discovers the terrifying truth behind the computer-governed caste system that dictates every citizen's fate. Kit, a brilliant youth, finds himself caught in the machinery of a society divided into zones of proles, bourgeois, and elite technicians. As he uncovers the conspiracy controlling the nation, he must navigate a landscape of surveillance and social engineering to challenge the central authority. The narrative follows his struggle to maintain autonomy against a system that treats human lives as disposable data points.

This novel is widely regarded as a prescient work of speculative fiction that anticipated many modern concerns regarding surveillance and algorithmic control. Critics often highlight the stark, uncompromising nature of Westall's world-building, which avoids easy resolutions in favor of a gritty examination of power. The book's status as a Carnegie Medal runner-up underscores its enduring impact on the genre and its ability to resonate with readers across generations. Its reputation rests on the author's capacity to weave complex social critique into a compelling narrative of rebellion against an all-encompassing state.