[[quoteright:350:http://static.tvtropes.org/pmwiki/pub/images/zygons_3776.jpg]]
->''"Oil? An emergency? Ha! It's about time the people who run this planet of yours realised that to be dependent on a mineral slime just doesn't make sense."''
-->-- '''The Doctor''' doesn't agree with the Brigadier's assessment.

The TARDIS arrives on Earth following the Brigadier's summons. The Doctor decides to wear a Tam o' Shanter for the occasion, decorating Harry and Sarah Jane with his scarf and hat respectively. The Brigadier is sporting a fancy kilt. Once he and the Doctor are done making fun of each other, they get down to business: UNIT are investigating a series of attacks on North Sea oil rigs from a temporary base in the Scottish village of Tulloch. The Doctor discovers that the attacks are the work of a giant lactating cyborg dinosaur in Loch Ness, controlled by an alien race called the Zygons from their ship beneath Loch Ness. The Zygons can imitate the form of other life-forms whom they have imprisoned on their ship, and are currently impersonating many of the village locals in order to infiltrate the UNIT base. When Harry gets shot in the head, the Zygons kidnap him from the hospital, copy his body and attack Sarah Jane wirth a pitchfork in order to get to the Doctor.

The Zygons plan to use their shapeshifting abilities to take over the world as a substitute for their own dead planet, but the Doctor releases the prisoners and destroys the ship, single-handedly killing all but one of the Zygons. The survivor, Broton, makes his way to London disguised as the Duke of Forgill. He plans to destroy a World Energy Conference using the cyborg, known as the Skarasen, and use this show of strength to persuade the world to give in to his demands. However, before he can do so, he's shot by UNIT in the middle of London, and the Doctor feeds the tracking device to the Skarasen. With no more control, the Skarasen returns to the only home it knows -- Loch Ness. Harry decides he's had enough of this time-travel lark and remains on Earth while the Doctor and Sarah depart for adventures new.

!!Tropes:

* AllThereInTheManual: Why the Caber is called that. According to the {{Novelization}}, he won a caber-tossing contest in the Highland Games ("caber" in this instance referring to a tree trunk).
* AlmostDeadGuy: Munro, before being rendered totally dead by the Zygon Caber.
* BalconySpeech: Lampshaded by the Doctor.
-->"You have to step out on a balcony from time to time and wave a gracious tentacle."
* BattleaxeNurse: Sister Lamont, who is actually a Zygon.
* BBCQuarry: The location the Zygon ship lands at in the final episode is explicitly stated to be a disused quarry.
* BookcasePassage: Sarah Jane Smith finds a passage behind a bookcase leading to the alien spacecraft under Loch Ness. She activates the opening mechanism by pulling out a book, [[VillainBall which she could only reach via portable steps fetched for her by one of the Zygons in human disguise]].
* BondVillainStupidity: Despite constantly assessing the Doctor as "too clever" and "dangerous," Broton and his cronies make only halfhearted attempts to kill him, and never stick around long enough to be certain.
* DistressedDude: Both Harry Sullivan and the Doctor end up being kidnapped in this story, and for once, it's not the girl companion -- Sarah Jane even rescues Harry Sullivan at one point!
* EverythingsLouderWithBagpipes: Angus the landlord plays "Flowers of the Forest" on the bagpipes. It turns out to be an omen (of sorts).
* FamilyUnfriendlyViolence: The Zygon Harry's attack on Sarah with a pitchfork and accidental impalement with it is one of the most disturbingly violent scenes in the history of the series, approaching a SlasherMovie in atmosphere.
* {{Foreshadowing}}/CerebusRetcon: In "Day of the Doctor" it is revealed that the destruction of the Zygons's homeworld mentioned in this episode was in fact an extremely early impact of the Time War on the Doctor's own adventures.
* ForcedPerspective: Used to achieve shots of characters leaving the Zygon spaceship in the final episode.
* ImmuneToBullets: Averted. Although Broton brags about the invincibility of the Skarasen, it turns out that Zygons are ''not'' immune to Earth weaponry. Benton wounds one, and the Brigadier even puts paid to Broton himself at the end.
* ImpaledWithExtremePrejudice: The Zygon impersonating Harry tumbles off the haystack after lunging at Sarah and is promptly skewered with his own pitchfork.
* LeftTheBackgroundMusicOn: Bagpipe music in the first establishing shot of the village turns out to be Angus the pub landlord deliberately trying to irritate the [=UNIT=] types who've taken over his establishment.
* MaliciousMisnaming: The Duke (actually Broton) deliberately calls the oil company manager Huckle "Heckle"
* ManInAKilt: The Brigadier.
** Also the Caber, the Duke's "Highland Games Champion" manservant.
* MyCarHatesMe: Whilst fleeing the Skarasen, the Doctor's UNIT Land Rover breaks down.
* NationalStereotypes: The very first line of the story mentions haggis. The last line is a joke about [[ThriftyScot Scots being mean]]. In between we have kilts, beards, bagpipes, second sight, and the Loch Ness Monster.
* OnlyKnownByTheirNickname: "The Caber." The Duke (actually Broton in disguise) tells Sarah that the Caber's real name is "very Gaelic" and she probably couldn't pronounce it.
* OrganicTechnology: The Zygons' spaceship. Only the interior appears organic, the exterior seems to be a metal hull.
* ThePiratesWhoDontDoAnything: Averted. We actually see Sarah typing out a few words of a news dispatch, for once.
* PutOnABus: Harry Sullivan decides he'll take the train back to London.
* ShownTheirWork: The Brigadier is wearing the appropriate tartan for his clan (the hunting Stewart tartan).
* StockNessMonster: [[Recap/DoctorWhoNSS2E3SchoolReunion "THE Loch Ness Monster!"]]
* TapOnTheHead: The Zygon disguised as Sister Lamont rather savagely bonks a UNIT soldier over the head with a rock. Somehow he survives without getting his skull split open.
* VoluntaryShapeshifting: The Zygons have the ability to transform themselves to resemble humans they've abducted.
* TheXOfY
* WhatCouldPossiblyGoWrong: Sarah Jane only agrees to get back into the TARDIS this time if the Doctor promises they'll go straight to London, five minutes before he left, as he confidently stated it could. He guarantees it. [[Recap/DoctorWhoS13E2PlanetOfEvil We all know this can't possibly go awry...]]
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