One of my guilty pleasures, when I need a rest from SF, is reading old detective stories. Sometimes, though, even these contain science-fictional bits. Sherlock Holmes himself meets weird science in "The Adventure of the Creeping Man" – investigating a chap who's hoping for rejuvenation through monkey glands (this was a popular fad of the 1920s) and is influenced to behave in strange ways which might be termed monkey business. "Oook, my dear Watson."
An early Agatha Christie novel, The Big Four, sees her regular detective Hercule Poirot battling an international cabal that plans to conquer the world. One of the Four, a French radiation scientist suspiciously reminiscent of Marie Curie gone bad, invents a death ray that wrecks US warships. Another is an evil Chinese genius all too obviously based on Sax Rohmer's Doctor Fu-Manchu. Needless to say, this gang is no match for Poirot's little grey cells, but it was a distinctly offbeat episode of his career.
Near the end of her own career, 43 years later, Agatha Christie invented another world-threatening conspiracy in Passenger to Frankfurt. Here the revived Hitler Youth cause widespread chaos but will be defeated (some time after the book ends) by a great scientist's "Project Benvo", aimed to produce a designer drug that makes everyone benevolent and nonviolent. Blimey. This idea of chemically induced nonaggression also features in Michael Innes's thriller Operation Pax, where it's treated as a horrific Thing Which Which Man Should Not Meddle. Quite right too.
Leslie Charteris injected SF into several stories about his roguish hero the Saint – such as The Last Hero, with a nasty "electron-cloud" weapon created by a genuinely mad scientist who gets his just reward: "I shot him like a mad dog." Later Saint adventures feature zombies, giant ants, the Loch Ness Monster, and some very strange inventions ... though none as implausible as the antigravity device that doesn't fit at all well into Mickey Spillane's raunchy thriller with the double-entendre title, The Erection Set.
Margery Allingham's The Mind Readers introduced her long-running sleuth Albert Campion to a genuine telepathy machine invented by schoolkids, using the newly discovered element nipponanium. The really prophetic part involves trawling classmates' minds for exam answers, anticipating the modern tradition of copying your homework from Wikipedia. More telepathic children – whose powers are here the side-effect of a kind of sleeping sickness – baffle Peter Dickinson's hard-headed series policeman Jimmy Pibble in Sleep and His Brother. These SF themes creep in everywhere.
Much more common in crime fiction is a touch of the supernatural. Ernest Bramah's blind detective Max Carrados meets a tasty example in "The Strange Case of Cyril Bycourt", where the unfortunate young Cyril is driven half mad by gruesome recurring nightmares of the Black Death – eventually traced by Carrados to the (then) new-fangled electric wiring which carries bad vibes from a generator built over an old plague-burial pit.
John Dickson Carr specialized in impossible crimes, usually committed in locked, sealed rooms. Often it seemed that vampires or other occult forces were the only explanation – until Carr's vastly corpulent detective Gideon Fell (based on G.K. Chesterton of Father Brown fame) explained how the trick was done. But there's one unforgettable Carr mystery, The Burning Court, in which after strong hints of witchcraft the impossible murder is duly explained away – whereupon a chilling final chapter breaks the rules of classic detective fiction by revealing that it was $SPOILER all along....
David Langford gazed at the reader through half-closed eyes: "You see, Watson, but you do not observe. You know my methods. Apply them."