This serial was originally composed of six episodes, but it was deemed too short of content and reduced to five at the last minute. Producer Peter Bryant ordered Haisman and Lincoln to abandon writing the sixth episode and script editor Derrick Sherwin rewrote the fifth episode to provide a conclusion. Haisman and Lincoln were not informed of this, or of the BBC's merchandising of the Quarks, which led to their refusal to write for the series again. Subsequently, an additional episode had to be penned for the following "The Mind Robber", also making that story five parts.