Adam Knight (John Turner) is a modern day slayer of dragons although the 'dragons' in this series are blackmailers, confidence tricksters and thieves.

Knight has drifted from one job to another without much satisfaction until he decides to set up his own business. He takes out an advertisement in a newspaper which reads: "Knight Errant '59. Quests undertaken, dragons defeated, damsels rescued. Anything, anywhere, for anyone, so long as it helps. Fees according to means" and he soon has no shortage of customers. Aided and abetted by ex-'Daily Clarion' journalist Liz Parrish (Kay Callard) and young writer Peter Parker (Richard Carpenter) Knight sets up an 'adventurer for hire' agency called 'Knight Errant.' The series returned the following year as 'Knight Errant '60' and later there are a few cast changes. Adam takes on Toby Hollister (William Fox) as his new business consultant before leaving himself to be replaced by publisher Stephen Drummond (Hugh David) at which point the series title changed to 'Knight Errant Limited'.

Many guest stars in the 1960 series would go on to become regular screen faces and included Dermot Kelly ('The Arthur Haynes Show'), Roger Delgado (The Master in 'Doctor Who'), David McCallum ('The Man From U.N.C.L.E.'), Dudley Sutton (Tinker in 'Lovejoy'), Sam Kydd ('Orlando'), Paul Eddington ('Yes Minister' and 'The Good Life'), Brian Pringle ('The Dustbinmen'), William Roache, Peter Adamson and Doris Speed (all of whom were about to embark on a new series called 'Coronation Street' (as Ken Barlow, Len Fairclough and Annie Walker)), Peter Vaughan (Harry Grout in 'Porridge') -and in an episode broadcast on Tuesday 12th April, 1960, a photographer was played by a young actor by the name of Michael Caine. Richard Carpenter took the same career path of his character by becoming a writer off screen and created (among other series) 'Catweazle.' The theme of a modern-day crusader and champion of the underdog for hire was revisited in the 1980s US series 'The Equalizer.'