Summoned back to Earth by the Brigadier, the Doctor investigates a series of mysterious attacks on North Sea oil rigs. Centre of the attacks is the village of Tullock, very close to Loch Ness. The Doctor discovers that the cause of these attacks is the Loch Ness Monster, in reality a Skarasen, a creature half animal, half machine, created by the Zygons, an alien race whose crippled spaceship has rested on the bottom of the Loch for centuries. Unable to return home (their planet was destroyed), the Zygons plan to take over Earth and turn it into a new home planet for their race. Able to change form and take on the physical appearance of Humans, the Zygons have infiltrated the village. The Doctor exposes the Zygons and flees their captives, including the local Duke. The Brigadier blows up the Zygon ship but Broton, leader of the Zygons, now in London, threatens to destroy a World Energy Conference. He is stopped by the Doctor. The Skarasen survives to swim happily back to Loch Ness.
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The Doctor:  Tom Baker
Sarah Jane Smith: Elisabeth Sladen
Harry Sullivan: Ian Marter
Brigadier Lethbridge-Stewart: Nicholas Courtney
Sergent Benton: John Levene

Guest Appearances:
Duke of Forgill/Broton: John Woodnutt
Munro: Hugh Martin
Huckle: Tony Sibbald
Angus McRanald: Angus Lennie
The Caber: Robert Russell
Radio Operator: Bruce Wightman
Sister Lamont: Lillias Walker
Corporal: Bernard G. High
Soldier: Peter Symonds
Zygons: Keith Ashley, Ronald Gough

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Producer: Phillip Hinchcliffe
Script Editor: Robert Holmes
Writer: Robert Banks Stewart
Director: Douglas Camfield
Designer: Nigel Curzon
Costume: James Acheson
Make up: Sylvia James
Visual Effects: John Horton, John Freidlander
Music: Geoffrey Burgon

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Novelized as "Doctor Who Terror of the Zygons" by Terrance Dicks (0 426 11041 2) first published by Tandem as "Doctor Who and the Loch Ness Monster" in 1976 with cover by Chris Achilleos. New edition by Virgin Publishing Ltd in 1993 with cover by Alister Pearson. Target library number 40.
The American Pinnacle edition was number 6 in the series and was released in June 1979. By January 1989, inor alterations to the cover. Cover art by David Mann.

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Released as "Terror of the Zygons" (BBCV 4186) first released in November 1988 and in US (CBS/FOX catalogue # 5422) with photomontage cover.Re-released in episodic format in 1999, PAL (BBC Video catalogue# 6774) and NTSC (Warner catalogue #E1410) with new photomontage cover.

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The bagpipes heard in the story were played by Angus Lennie, who played 'McRannald'.
Ian Marter leaves at the end of the story, but returns for a guest appearance in "The Android Invasion".
Douglas Camfield returned to Doctor Who. Camfield had not worked on the program since 1970, when he experienced heart trouble while making "Inferno".
This is the last story to feature the Brigadier until "Mawdryn Undead".

All four episodes exist in color on videotape.

Studio recording in Stage TC3. (7-8 April 1975)
Studio recording in Stage TC4. (22-23 April 1975)
Climping Beach, Climping, West Sussex. (17 March 1975)
Ambersham Common, South Ambersham, West Sussex. (17-19 March 1975)
Hall Aggregates Quarry, Storrington, West Sussex. (18 March 1975)
Charlton, West Sussex. (20 March 1975)
Furnace Pond, Mill Lane, Crabtree, West Sussex. (21 March 1975)
Millbank Tower, Millbank, London, SW1. (Date Unknown)

Episode 1 - When the Brigadier summons the Doctor back from Jupiter, he complains that he's just been dragged 270 million miles. (It's actually more like 370 million miles.)
Episode 2 - When the Doctor and the Brigadier are examining the soldier who was crushed to death by the Skarasen, you can see one of his (the soldier's) fingers move slightly. Pretty hard to do if you're dead!
Episode ? - When the duplicate Harry steals the transmitter from Sarah, the wound on his forehead is very light in colour (as if it had almost healed). When he runs out the door it is suddenly very dark red.