John D. MacDonald
Published in Doc Savage, December 1946.
"Benton Walters had to slug his employer to get the job - and that was only the beginning. This screwy work in which nobody gave his right name and deal with invisible enemies made life in the army seem like a picnic."
Getting hired for the job and immediately operating on his own, Benton Walters is first given a series of assignments investigating apparently unconnected matters, each of which test his ability to think on his feet and prove his creativity. He is told later each of these tasks are part of an overall mission to stop "certain Nazi characters in Spain who struggle for a revival of the Fatherland via new war by disease".