Here it is – The Possessed Hand
There’s a neurological disease known as ‘alien hand syndrome’ that causes the person afflicted to be completely unaware of what one of their hands is doing.
Yes. Anyone with the disease is estranged from the behavior of the hand (or limb) which will behave “capriciously and without direction” from its owner. Sufferes have been awaked by their random hand stroking their face, pulling an ear or messing with their privates.
There are a number of etiologies that could be involved, from certain Alzheimer’s brain lesions, stroke, dementia and uncategorized “damage”. A fall, a blow to the head. Regardless of cause, the hand does its own ‘Thing”.
Instead of the angry severed hand, though, a great plot I’ve only seen a few times would involve a connected hand with a politically incorrect mind of its own. Instead of a middle finger popping out, an involuntary Incredible Hulk style power-choke grip lifts a conveniently deaf convenience store clerk over the counter while the owner demands half the two-fer price for a single bag of Doritos.
Take a serial killer who is by day a pleasant enough high speed interface engineer who gets driven around at night by his hand to commit murder most foul. It could be written from the Grisham-Turrow-Gardner angle, the Cornwell-Garritson angle, a pick an author whodunnit, a Laura Levine-esque humor take or a Noir shamus riff titled The Upper Hand.
Bonus – Give the hand owner Tourette’s to go with and the comedy at the convenience store explodes. I’ll spare you a demonstration.