The notion of presupposition, well-known in logic, is applied to the study of manipulative techniques in detective fiction. More specifically, the study focusses on pragmatic presuppositions which can be defined in terms of assumptions the speaker/writer makes about what the hearer/reader is likely to accept without challenge. A number of classical British detective stories, novels and plays are examined that employ dubious techniques which, though not banned by the rules of the genre, seem to break the credit of trust the readers confer on the author. The paper suggests that such cases, however miscellaneous they may be, fall under a single category involving manifold violations of the readers' pragmatic presuppositions.