sentences:  The storyline for the four-part story was commissioned on 31 January 1975 and the full scripts on 4 February 1975.Written by Barry Letts, the storyline for this four-part story was commissioned on 21 January 1975.It was based on an audition piece for the role of Sarah Jane Smith that Letts had written in 1973 and was initially known as "Time Lord Story".