The paper analyses the scheme whereby provincial chief constables, encouraged by the Home Office, could call in the help of Scotland Yard's experienced detectives to investigate serious cases, especially murder, that were considered to be beyond the capacity of the local force to solve on its own. While the scheme was on balance successful in that more than 50% of such call-outs between 1919 and 1928 resulted in convictions, it is suggested that its significance extended beyond the mere profit and loss accounting approach. For the arrangements cast a mirror on many of the conflicts and some of the developments in policing during this period. Thus they illuminated the tension between respect for local, even if inexperienced, police autonomy, on the one hand, and efficiency and expertise on the other; or, more broadly, between constitutional localism and central governmental direction of policing in England and Wales. But with the press campaigning for more efficient use of the scheme, its arrangements actually attested to the favourable prospects, in the late 1920s and 1930s, for inter-force cooperation in the form of national crime prevention schemes (such as that designed to intercept cross-country ‘motor bandits’). It was thus one of the unacknowledged elements in the forcing house for organisational change experienced by British policing before the war.