It was designed as a vehicle for Detective Chief Inspector Charles Barlow and Detective Inspector John Watt (played by Stratford Johns and Frank Windsor, respectively) from the police series Z-Cars, which had just finished its original run in December 1965 (no new episodes were produced in 1966 but it was revived in a different format the following year).

Joining them in the early series was Robert Keegan as Blackitt, the police station sergeant from Z-Cars, now retired and acting as a freelance helper.

[2] The 1968 episode "Unfinished Business" saw Barlow reunited with his former boss from Z-Cars Detective Chief Super Intendentent Robins (John Phillips).

[3] The first two series continued the trend set by producer David Rose with Z-Cars and transmitted the majority of episodes live.

The characters of Barlow, Watt and Hawkins were promoted and moved to the Southeast of England in a new series set in the fictitious town of Thamesford.