The premise of the novel I found very compelling, because I don't know of any other novel that deals with the nature of Britain's wartime relationship (economically and diplomatically) with Francoist Spain.
In Sampson's novel, you have Harry Brett, a "traumatized veteran of Dunkirk turned reluctant spy" in Britain's Secret Intelligence Service (SIS), who is sent to Spain under diplomatic cover in mid-summer 1940. While in Spain, he tries to ingratiate himself with a shady Englishman involved in some mining extraction enterprise which is proported to sizeably increase Spain's gold reserves (which had been depleted by Stalin during the civil war there) --- and thus strengthen Franco's bid to speed up Spain's recovery.
Sampson has a lot of interconnected backstories among the various main characters of the novel. Not all of them, however, helped to make the novel more compelling to me. That is what frustrated me the most. The subject matter was really interesting stuff(!!!) But the development of the plot and its denouement left a lot to be desired.