A.A. Milne is, of course, best known as the creator of Winnie the Pooh and the other inhabitants of the Hundred Acre Wood, but he dabbled in mystery fiction from time to time. His novel The Red House Mystery is well-written, thought the plot is somewhat pedestrian. (Alexander Woollcott famously called it one the of the three best mysteries of all time, which suggests to me that Woollcott only read three mysteries.)
As "A Perfectly Ordinary Case of Blackmail" opens, Sir Vernon Filmer, a politician of long service and some repute, as well as a rather superior manner, arrives at the office of his very proper solicitor, Cedric Watherston. Filmer has a problem - as the title of the story indicates, he's being blackmailed. Watherston, quite naturally, wants to know none of the particulars. He does know a man who specializes in such cases, another solicitor whose morals are rather more flexible than Watherston's own.
(Watherston would normally never have associated with such an individual, but he'd been very useful when they were both prisoners of the Kaiser in 1917.)
Scroope was, in fact, a very useful indivdual, and soon he had the story out of Sir Vernon. Many years before, before the Great War even, a man had mistaken Sir Vernon for his wife's lover and viciously attacked him. Sir Vernon fought back and had killed the man, after which he and the man's body were discovered by Sir Vernon's friend. It looked damning. The fight had been brutal, and the wounds inflicted on the dead man could easily have been interpreted as deliberate murder.
Being a professional in such matters, Scroope cheerfully begins making his preparations, and with the clever way he works it, the guilty party - or parties - are guaranteed to get what they deserve.
This was a delightful comic story. Milne sketches the characters quickly but clearly, and his light touch lends an air of humor to the whole enterprise. The ending may not be strictly legal, but Scroope (and the reader) will no doubt find it just and proper.
I read this story in Masterpieces of Mystery: The Golden Age, Part 1, edited by Ellery Queen. I've read a couple of other volumes in the series (out of 20), and I'm working my way through The Golden Age, Part 2 right now. They're all excellent, and generally can be found at reasonable prices.