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It is June 1917. DDI Ernest Hardcastle, head of the CID for the Whitehall division of the Metropolitan Police, is called to investigate the murder of a very senior civil servant, Sir Nigel Strang.

Given that Sir Nigel was the permanent secretary at the Ministry of Munitions, Hardcastle wonders whether the German Intelligence Service might be responsible. Then another murder, of a man on a bus near Scotland Yard, leads Hardcastle to a woman working at Woolwich Arsenal who is suspected of passing information to the enemy. But is there a connection? Hardcastle, aided by Detective Sergeant Charles Marriott, must find out . . .