Wolf Hook by Michael Wallace
Jim Heydrich is the Canadian-born nephew of one of the most feared men in the Gestapo. When his family returns to Germany just before the war, Jim arrives in the Third Reich as a young, sensitive theater student, both protected by and encumbered by his famous relation.
Resisting an invitation to join the Nazis, he instead finds himself a member of an English-language theater troupe working in Occupied Europe. Unbeknownst to Jim, the leaders of the theater troupe, Nigel Burnside and his sister Margaret, are not the English fascists they appear, but members of the British Secret Service, using the theater troupe to recruit spies from among the Anglophile German officers who come to their productions.
Disillusioned with both sides of the war, Jim is trying to defect to neutral Ireland when he stumbles into one of Nigel and Margaret’s most closely held secrets–a Hungarian physicist they are smuggling out of Europe. While trying to extricate himself from this unwelcome knowledge, he both manages to draw the attention of the Gestapo and to convince the British Secret Service that he is a Nazi spy, a threat to their plans who must be eliminated.