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.

Tom Baker continued his role as The Fourth Doctor along with Sarah Jane Smith (Elisabeth Sladen). Harry Sullivan, played by Ian Marter, departed in Terror of the Zygons and reappeared in The Android Invasion as a guest star.

Nicholas Courtney returned as Brigadier Lethbridge-Stewart in Terror of the Zygons having last appeared in Robot. John Levene made his final appearance as Sergeant Benton in The Android Invasion after six years in the role.

1.  Terror of the Zygons - Four Episodes

    Synopsis: On an oil rig off the coast of Scotland, high-pitched beeping
    echoes through the structure, and it begins to break apart, collapsing
    into the North Sea. The Fourth Doctor, Harry and Sarah hitch a ride
    with the Duke of Forgill, a local landowner, to the town where the
    Brigadier and UNIT have set themselves up in an inn. The Brigadier is
    talking to Huckle, an official of Hibernian Oil, who owned the rig.
    Huckle complains that three such rigs have been destroyed in the last
    month.

2.  Planet of Evil - Four Episodes

    Synopsis: The TARDIS picks up a distress call and the Doctor and Sarah
    arrive on the planet Zeta Minor. There they discover that a Morestran
    geological expedition has fallen prey to an unseen killer and only the
    leader, Professor Sorenson, remains alive.

3.  Pyramids of Mars - Four Episodes

    Synopsis: In Egypt in 1911, Marcus Scarman excavates a pyramid and finds
    that the door to the burial chamber is inscribed with the Eye of Horus.
    Scarman's Egyptian assistants panic and flee at the sight of the glowing
    hieroglyph, leaving the Professor to enter the chamber alone. As he holds
    a light up to see the undisturbed tomb, he is blasted by a ray that
    emanates from a seated and cowled figure.

4.  The Android Invasion - Four Episodes

    Synopsis: The Doctor and Sarah find themselves in the English village of
    Devesham. But the village seems deserted, except for mysterious space
    suited figures and their own doppelgangers. 

5.  The Brain of Morbius - Four Episodes

    Synopsis: Years ago, the renegade Time Lord known as Morbius was executed.
    When the Fourth Doctor and Sarah arrive on the planet Karn, they discover
    that Dr Solon is creating a body so that Morbius may live again. 

6.  The Seeds of Doom - Six Episodes

    Synopsis: Arctic explorers uncover two giant seedpods from another time
    and place. Ruthless plant-lover Harrison Chase decides he must have them
    for his rare collection, but the pods may spell the end of all non-plant
    life on Earth.