Context : Horror of Fang Rock was a late replacement for the scripts Terrance Dicks had originally submitted, a vampire-based tale entitled The Vampire Mutations, which was cancelled close to production as it was feared it could detract from the BBC's Count Dracula a high-profile adaptation of Bram Stoker's classic novel Dracula, which was due for transmission close to when the serial would have aired. 
Subject : Horror of Fang Rock 
Relation : series
Which series was Horror of Fang Rock in?