I read a short story, possibly sci-fi, and cannot remember the name. I did the same thing with a previous story, and got back the answer (“Piggy Bank”) in something under fifteen minutes, which is kind of scary. Nonetheless, if you are up for a challenge…
The story was about a person captured as a POW. He kept saying that the duty of a prisoner was to continue the fight. So he hit on the strategy of making up an imaginary spy, who was sabotaging the foe from behind enemy lines. So he lets the name of the imaginary spy (which I think was the title of the story) slip, and the enemy begins to attribute all kinds of random accidents and crashes to the imaginary spy. I remember a passage where the enemy shows the POW an interrogation of a different POW, who figures out what is going on and stonewalls his response when asked about this imaginary spy and thus confirms in the mind of the enemy that it is all real.
The story ends up with the enemy releasing the POW after surrendering, because they cannot figure out how to fight this imaginary spy.
This sounds very much like something by Eric Frank Russell, in particular his novel Wasp, although the plot differs slightly from your description (for one thing, in length – it’s definitely not a short story).
To quote from Wikipedia’s piece on EFR:
Wasp does feature the involved sabotage activities of a lone spy on an enemy world, but there’s nothing about him being a POW at the start or having to improvise – he went to the world with the intent of sabotaging, and only ends up as a POW at the end.
It’s possible this might be some other EFR short story. I’d have to look through my collections. Russell is one of the great underappreciated SF writers, as Jack Chalker always maintained.
It’s Eric Frank Russell but it’s Next of Kin aka The Space Willies, not Wasp, although there are similarities.
The invisible companion is called Eustace, and the POW convinces his captors that every Earthman has one…
Sure sounds like it to me, even though I seem to remember the shorter version title as “Plus X”. “The Space Willies” was the full novel. I must confess that I’ve always loved Russell. The humans were always brass-balled smart asses and the aliens didn’t stand a chance.
I would also recommend “Three to Conquer” and “Wasp”.
