August, 1939. As war looms, Detective Inspector Walter Hardcastle is following in the footsteps of his father, Ernest, and progressing through the ranks of the Metropolitan Police. Summoned to the office of Charles Marriot, Deputy Assistant Commissioner of CID at New Scotland Yard, Walter learns that he has been promoted to a senior role at V Division, and is plunged into a top secret case when burglaries occur at the homes of those working on a new type of war submarine.

Could a German spy be behind the break-ins? When one of the burglaries ends with a double murder, the stakes get higher for Walter. Can he track down an enemy agent on the streets of London while also resolving another disturbing case?