Using the code name "Mount", he was one of the group, which he supported with his own money, led by Monsignor Hugh O'Flaherty and a French diplomat François de Vial who helped conceal some 4,000 escapees, whether Jews or Allied soldiers, from the Nazis: 3,925 survived the war. Their story was portrayed in the 1983 film "The Scarlet and the Black", starring Gregory Peck. He also played a key part in a plot in 1940, which involved the Pope and certain German generals, to overthrow Hitler. Major Sam Derry, in his book "The Rome Escape Line", described meeting Sir D'Arcy in the Vatican in 1943:Unruffled poise... Seldom have I met any man in whom I had such immediate confidence. He welcomed us warmly, yet I found it impossible to behave with anything but strict formality. Apart from the restraining influence of my clothing was disguised as a monsignor I was almost overwhelmed by an atmosphere of old-world English courtliness and grace which I had thought belonged only to the country-house parties of long ago. Sir D'Arcy was spry, trim, a young sixty, but he had spent years enough in the diplomatic service to develop an astonishing aptitude for creating around himself an aura of all that was most civilized in English life. I felt as though I had returned home after long travels, to find that royalty had come to dinner, and I had to be on my best behaviour.