Plus X
by Eric Frank Russell, Astounding Science Fiction (June, 1956)
collected in “4 for the Future” ed. Groff Conklin (Pyramid, 1959), pp. 108-160.
Russell’s impish SF novella pits a lone POW caught deep behind enemy lines, armed with nothing but his wits (and thence an imaginary co-conspirator!), against the entire alien war effort. Leeming’s captors might be a tad credulous but this is terrific fun.