We Have Already Seen...
...after an assassination attempt is made on Doc Savage and his team, they discover the now-dead killers were Nazis sent to prevent the adventurers from interfering in "Operation Siegfried", involving a German they had encountered years before...
Note: It had long been established back in the original pulp magazines that Doc met the future members of the Amazing Five while they were all incarcerated during World War I in German POW camp "Loki"...which they escaped from!
Think League of Extraordinary Gentlemen, but on steroids!
He had also penned a treatment for a sequel (Doc Savage: Arch-Enemy of Evil) to the 1975 George Pal movie Doc Savage: the Man of Bronze, and scripted several pastiches with a thinly-disguised Doc Savage named Doc Caliban along with an equally-thinly disguised Tarzan, including this obvious tribute to the Bantam reprints with a kool Gray Morrow-doing-James Bama cover...
Trivia: AFAIK, Escape from Loki is the only Doc Savage prose novel to not be reprinted in any form!