The plot is loosely based on the short story "Greenshaw's Folly" (1956), which was first published in the newspaper Daily Mail from 3 to 8 December, 1956. It was collected in book form in the short story collection "The Adventure of the Christmas Pudding and a Selection of Entrées" (1960), the only Christie first edition published in the UK that contains stories with both Hercule Poirot and Jane Marple.
The poisoning aspect of the case and the request of pilocarpine as an antidote was not based on "Greenshaw's Folly" (1956), but the short story "The Thumb Mark of St. Pete" (1928), where Jane Marple investigated the murder of her nephew-in-law Geoffrey Denman.