Compared, not only with Sherlock Holmes and Father Brown, but with Nero Wolfe or Dr Fell or Lord Peter Wimsey, Poirot is a distinctly cardboard character, an obvious artefact. Agatha Christie herself prefers Miss Marple, and her new book, Passenger to Frankfurt, contains neither of them.

The fact remains, however, that Poirot, like a survivor from an almost extinct race of giants, is one of the last of the Great Detectives: and the mention of his name should be enough to remind us how much pleasure Agatha Christie has given millions of people over the past fifty years….

So what is it, this quality which Agatha Christie possesses and so many imitators have lacked?

The secret does lie partly in her plots. The Murder of Roger Ackroyd, Murder on the Orient Express, The ABC Murders and her other classic tours de force deserve their fame. If they seem...