London in the autumn of 1939: the early months of World War 2.

Hitler has just conquered Poland, but life in London continues much as it did in peacetime, albeit a little more restricted since the introduction of petrol rationing. No bombs have been dropped on the city as yet, but the population go about their daily business under the constant threat of German air raids, and a blackout remains in force at night.

For Scotland Yard and the criminal fraternity, however, it is business as usual. When a pawnbroker's assistant is wounded by a gunman wearing a gasmask, it appears to be a straightforward case of a bungled armed robbery, but as Detective Inspector Hawker of the Flying Squad proceeds with his investigation, the more facts he uncovers the more confusing the affair becomes. A red-headed soldier, a missing gun, a dead cat, an empty violin case and a damaged violin are only a few of the threads that have to be unravelled before he can wrap up the case.