A Miss Marple Nightcap: The Thumb Mark of St. Peter by Agatha Christie

Things get a little wild when Miss Jane Marple herself tells the
story in this, the final installment of the Tuesday Night Club.
We've got carp; we've got haddock; we've got insanity; we've got
the power of prayer. We may even have (gasp) true love? Has a
romance been blossoming amidst these weekly tales of mayhem and
intrigue? Leave it to Miss Marple to ferret it out... and to put it
in its place.