London's craftiest and boldest detectives, Arthur Bryant and John May, are back in this deviously twisting mystery of black magic, madness, and secrets hidden in plain sight. When a young woman is found dead in the pews of St. Bride's Church--alone and showing no apparent signs of trauma--Arthur Bryant assumes this case will go to the Peculiar Crimes Unit, an eccentric team tasked with solving London's most puzzling murders. Yet the city police take over the investigation, and the PCU is given an even more baffling and bewitching assignment. Called into headquarters by Oskar Kasavian, the head of Home Office security, Bryant and May are shocked to hear that their longtime adversary now desperately needs their help. Oskar's wife, Sabira, has been acting strangely for weeks--succumbing to violent mood swings, claiming an evil presence is bringing her harm--and Oskar wants the PCU to find out why. And if there's any duo that can deduce the method behind her madness, it's the indomitable Bryant and May. When a second bizarre death reveals a surprising link between the two women's cases, Bryant and May set off on a trail of clues from the notorious Bedlam hospital to historic Bletchley Park. And as they are drawn into a world of encrypted codes and symbols, concealed rooms and high-society clubs, they must work quickly to catch a killer who lurks even closer than they think.