England, June 1944. Exactly one week after the Allied armies invade Nazi occupied Normandy, Hitler strikes back with his Vengeance Weapons: the terrifying V-1 Flying Bombs. In August, one of these Doodlebugs falls on an Anglican convent near Camberwell, killing three nuns. DS Suzie Mountford is called out with her boss, and secret lover, Tommy Livermore, when it is discovered that one of the dead nuns is not quite what she seems.
The Reserve Squad is plunged into a situation of feuding families, strongly held political views that could be a threat to Britain and a Nazi plot to bring about an endgame to the war.