It's 1940, and Britain's war against Nazi Germany isn't going well. The British Expeditionary Force has been evacuated from Dunkirk back to England, and while the evacuation was a success, it was also a retreat. Hitler is planning an invasion of Britain, and the British military and political leadership really don't know how they're going to mount an effective defense.
Col. Sir John Graham, "Gray" to his friends and family, is a military intelligence officer--and also a high-ranking member of an occult group, practitioners of the Britain's traditional, ancient rites. This group, the Oakwood group, and other occult groups in Britain, are planning their own response to Hitler's invasion plans. This includes responding, more directly than conventional military forces can, to Hitler's own occult assault on Britain and Britain's military forces.
In addition to his occult colleagues, Graham has another dear friend--Prince William, Duke of Clarence, the (fictional) twin brother of King George V's youngest son, John, who died at age fourteen. This close friendship is going to lead to some serious complications as the Oakwood group works to organized a unified British occult response to Hitler.