Plot hole: One glaringly obvious script blunder is that the murder of Fitch and Parker is supposed to have taken place "six years previously" (in the summer of 1971, the date established the pre-title sequence) yet Haydon has been in prison for "seven years and five months" according to his daughter. This makes no sense, unless the viewer is to assume that the "present day" sections of the story take place in early 1979. (The episode was first broadcast in February 1978).
Plot hole: The chronology of this episode makes no sense at all in places. For example, the pre-titles sequence makes it clear that it is the summer of 1971, and Doyle mentions to his sergeant that CI5 is being set up. Yet series creator Brian Clemens' own writer's guide specifically stipulated that the CI5 wasn't even conceived until November 1971.