The first two series were shot mainly in a studio on videotape with filmed location inserts; a standard method of the time but one which suffered from jarring differences in picture quality between interior and exterior scenes.

These episodes starred George Sewell as DCI Alan Craven and Roger Rowland as DS Bill North.

By the 1974 series Bill North had been axed, having had a nervous breakdown, though he returned for one episode later in the run as a Detective Inspector in CID.

Played by Morris Perry and Paul Eddington respectively, Moxon and Strand were similar: supercilious civil servants who kept an unwanted eye on the detectives and their budgets.

They sometimes manipulated them in order to pursue obscure matters of state, such as in the first series episode "Reliable Sources", where Moxon connives to have Eden sidelined into a Home Office working party role.

[6] Notable actors appearing included: Paul Darrow ("Smokescreen"), Dino Shafeek and Windsor Davies ("The Promised Land"), Garfield Morgan and Rula Lenska ("Something About a Soldier"), Jacqueline Pearce ("Catherine the Great"), Cyd Hayman ("Rendezvous"), Susan Jameson as Detective Sergeant Mary Holmes ("Date of Birth", "Intercept", and "Downwind of Angels"), John Bindon ("Intercept"), Peter Bowles and Janet Key ("Downwind of Angels"), Nicolette McKenzie ("Diversion"), Denis Lill ("Diversion"), Kenneth Colley ("Date of Birth"), Valerie Leon ("Sounds Sinister") June Brown ("Entente Cordiale"), Annette Crosbie ("The Other Man"), Michael Gambon and Nadim Sawalha, ("Hostage"), Dennis Waterman and Stephanie Turner (as brother and sister in "Stand and Deliver"), Stephanie Beacham ("Threat"), Richard Marner ("Polonaise"), Roger Lloyd Pack ("Red Herring"), Gareth Thomas ("Alien"), Tony Haygarth ("You Won't Remember Me") and Walter Gotell ("Intercept").