The Thin Blue Line was set in the police station of the fictional English town of Gasforth. One of the main themes was the rivalry between the uniformed squad led by Inspector Fowler (a sort of protagonist figure) and the CID led by Detective Inspector Grim (often Fowler's antagonist, though they were on the same side of the law). DI Grim provided much of the humour through his mis-use of the English language. Episodes frequently saw the uniformed branch and CID locking horns over similar, or in some cases the same, issues while having conflicting views or methods of operation. Generally the uniformed section triumphed over the Detectives, although not without their own foibles.
The other comedic plotlines centred around the characters below and their peculiarities, although other PCs and staff were usually visible in the background and at meetings. (These ancillary characters' visibility is more obvious in the first series. In both series they generally have no speaking parts).
Main Article: List of episodes