"Carson's Conspiracy" (1984) is one of the slenderest Sir John Appleby mystery novels, and was published in Michael Innes's 78th year. The only novel from this prolific author to follow it was "Appleby and the Ospreys" (1986).
Sir John, retired Commissioner of the Metropolitan Police (Scotland Yard) no longer strays very far from home. He tells his friend, the local Chief Constable, that he is more of a Mycroft than a Sherlock now. However, he still occasionally lunches with the neighbors, and when this novel opens, Sir John and his wife are meeting their new neighbors over a slightly pretentious lunch of scallops and champagne.
Carl Carson, the "newish and rather unattractive" neighbor spends lunch fretting over whether his wife will do or say the wrong thing. He is a nouveau riche financier with some shady dealings in the City, and mistakes Appleby for someone "who had been high up in the Inland Revenue." His wife, Cynthia refers to Appleby as the "Commissionaire."
Really, the snobbery and counter-snobbery of the new versus the old gentry are what make this novel sparkle. In his later Appleby novels, Innes no longer relied as heavily on character eccentricities, surreal settings, or dense plotting. Instead, his readers scud merrily downstream on waves of witty, ironic dialogue, literary allusiveness, and an undertow of murderous intent.
The plot is transparent. If nothing else, the title gives it away and the first narrator is Carson, himself. His wife seems to have gone seriously round the bend and is chatting up their new neighbors with the exploits of an imaginary son. Carson, who sees serious financial problems looming dead ahead, decides to kidnap his nonexistent son, who is supposedly returning from America, convert his capital into ransom money, then disappear as another victim of the kidnapping plot---with the loot, of course.
His problems begin, although he is not aware of them at first, when Carson has to find someone to pose as his imaginary son. Every one is taken in by the conspiracy, including the Applebys, until a sudden, violent twist at the very end of the book.
This is a quick, enjoyable read, and don't believe Sir John when he tells the Chief Constable that "Mycroft has retired." He still has one novel to go, and his sharp-edged wit and elegant sleuthing will live on for a long while in the hearts of his readers.