British detective fiction and spy fiction, descended from nineteenth-century adventure narratives, come of age in the twentieth century. Both emphasize action over character, and coincidence over probability; both qualify as thrillers. But despite their similarities, detective and spy genres have asymmetrical relations to each other. To view them as equal and merely conventional forms of mass-market entertainment obscures their differing subjects, narrative structures, and ideological values.

The subject matter of each genre is quite distinct. Detective fiction foregrounds antagonisms that inevitably concern bloodlines and inheritance within the UK. Spy fiction focuses on affairs of state between Britain and other nations. Detective narratives, unthinkable without a corpse, presume the finality of death as a meaningful event. In spy fiction violent deaths of characters are incidental to international conflict, and not mysterious in that context. Plots and plans vary widely in the two genres. A murderer in detective fiction acts according to a planned sequence of steps, but actions (including murder) in spy fiction arise from changing global circumstances. Detectives work alongside the police, drawing on their services, while showing superior ratiocinative ability. Spies, in contrast, subordinate themselves to directives from handlers in a central bureaucratic agency. Such subordination tempts spies to make renegade decisions. A spy defying orders in John le Carré's The Perfect Spy (1986) sums up quandaries in the genre thus: “Sometimes our actions are questions, not answers.”