Terror of the Zygons is the first serial of the 13th season of the British science fiction television series Doctor Who, which was first broadcast in four weekly parts from 30 August to 20 September 1975. It was the last regular appearance by Ian Marter as companion Harry Sullivan, who would later return in The Android Invasion. It was also the last regular appearance by Nicholas Courtney as Brigadier Lethbridge-Stewart, although Courtney would return for guest spots as well.