Grave Mistake is a detective novel by Ngaio Marsh; it is the thirtieth novel to feature Roderick Alleyn, and was first published in 1978. The plot concerns the supposed suicide of a wealthy widow in a chic rest spa, and involves a rare, and famous, postage stamp. Set in and around the fictional village of Upper Quintern in England's Weald of Kent, it is the last of Marsh's "cosy" English village mysteries, and was followed by Photo Finish (1980), her final novel set in New Zealand, and Light Thickens (1982), her final novel with a theatrical setting. The book was well received and sold extremely well, as the author wrote in 1979 to a friend: "Grave Mistake seems to be beating all records in the U.S.A. It has sold 22,000 copies... & [is] still going strong" (quoted in Joanne Drayton's biogr