The show starred an American actor, Steve Forrest, as John Mannering, an antiques dealer and sometime undercover agent working in an informal capacity for the head of the fictional British Diplomatic Intelligence, Templeton-Green (Colin Gordon). He is assisted by Cordelia Winfield (Sue Lloyd) and David Marlowe (Paul Ferris).
In Creasey's original novels Mannering was British and, after the few first entries, married. In transforming him into a bachelor and casting a Texan in the role, the producers decided that 'The Baron' would be nicknamed after the cattle ranch once run by his grandfather that was described as being "200,000 acres (809.371 km2) 300 miles from Dallas". In the books he was a reformed jewel-thief (the first few novels described that "career" from Mannering's decision to steal to his going straight) whose criminal ties served him well in investigating jewel, art or antiques-oriented mysteries. For the TV series, Mannering's persona was depicted as absolutely straight with no suggestion of past criminality, a fact underlined by his co-opting by British intelligence. In the episode Red Horse, Red Rider it is revealed that Mannering was in the US Army in World War II in the Monuments, Fine Arts, and Archives program where he recovered art works from the Nazis. Following the war he owned three antique stores and was a "charter member of the jet set.