The thirteenth season of British science fiction television series Doctor Who began on 30 August 1975 with the story Terror of the Zygons, and ended with The Seeds of Doom. This is the second series to feature the Fourth Doctor, played by Tom Baker, with Philip Hinchcliffe producing and Robert Holmes script editing. In September 2009, it was ranked as readers' favourite season in Doctor Who Magazine issue 413.

Terror of the Zygons is the first serial of the thirteenth 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. The serial featured the last regular appearance by Ian Marter as companion Harry Sullivan and Nicholas Courtney as Brigadier Lethbridge-Stewart. The serial is the first appearance of the Zygons.