Terror of the Zygons is the first serial of the 13th season of the British science fiction television series Doctor Who, which was the first broadcast in four weekly parts on BBC1 from 30 August to 20 September 1975. It was the last regular appearance by Ian Marter as companion Harry Sullivan and Nicholas Courtney as Brigadier Lethbridge-Stewart. The serial is set in and around Loch Ness and in London. In the serial, the alien shapeshifters the Zygons plot to use their cyborg sea monster the Skarasen to take over the Earth after they discover their home planet was destroyed.