This story (which is actually just one adventure in the "Garry Halliday" series (qv)) was broadcast in the glorious era when most of the action was done live in the studio. Unfortunately, Elwyn Brook-Jones, who did a magnificent job of playing the silkily malevolent villain, The Voice, died part-way through the series. His replacement (whose name I do not remember, but who was introduced by a rather apologetic announcer) unfortunately did not look or sound much like Brook-Jones, and I suspect that the series then died without his contribution.
The replacement of an actor playing a major role in children's television may have been short-lived, but it preceded by several years William Hartnell's replacement as Dr Who, which usually gets credit for being the first.
The replacement of an actor playing a major role in children's television may have been short-lived, but it preceded by several years William Hartnell's replacement as Dr Who, which usually gets credit for being the first.