The one where the Brig wears a kilt.
The TARDIS arrives on Earth - Scotland, to be precise - 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.
The serial was actually filmed as the final instalment of the previous season, which is why it's one last hurrah for the Doctor-Sarah-Harry team and Season 12's transitional style. This was the last script that had been originally greenlit by Barry Letts and Terrance Dicks, and the next would complete the shift into the Gothic style Robert Holmes and Philip Hinchcliffe would become famous for.
- Almost Dead Guy: Munro, before being rendered totally dead by the Zygon Caber.
- Aristocrats Are Evil: The Duke. However, only because it's Zygon leader Broton having taken his form. The real Duke is imprisoned elsewhere. When freed, the trope is averted as he proves to be quite friendly.
- Balcony Speech: Lampshaded by the Doctor."But you can't rule a world in hiding. You've got to come out onto the balcony sometimes and wave a tentacle."
- Batman Can Breathe in Space: Not in space, but when Sarah and the Doctor are trapped in a hyperbaric chamber being drained of oxygen, he puts her and then himself in a trance to keep them from needing to breathe.
- Battleaxe Nurse: Sister Lamont, who is actually a Zygon.
- BBC Quarry: The location the Zygon ship lands at in the final episode is explicitly stated to be a disused quarry.
- Bookcase Passage: 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, which she could only reach via portable steps fetched for her by one of the Zygons in human disguise.
- Bond Villain Stupidity: 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.
- Brief Accent Imitation: Sarah answers a phone call in a silly Scottish accent to amuse herself - which turns out to be Mood Whiplash as the call was to tell her Harry has been shot. The Doctor also imitates a Scottish accent when mocking Forgill (not a very good one).
- Bulletproof Human Shield: While not actively using him as a shield, the man Harry tends to in the first episode is shot with a sniper rifle, without Harry getting hit. At least, until the rifle is aimed at him...
- Capture and Replicate: Zygon modus operandi.
- Chromosome Casting: Aside from Sarah, Sister Lamont is the only female character in the story.
- Cryptid Episode: The Doctor meets the Loch Ness Monster. Kind of.
- Distressed Dude: 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!
- Everything's Louder with Bagpipes: Angus the landlord plays "Flowers of the Forest" on the bagpipes. It turns out to be an omen (of sorts).
- False Reassurance: Sister Lamont tells the injured Harry, "Your worries are over now. You're going to be very well looked after." immediately before her Zygon colleagues disappear him.
- Family-Unfriendly Violence: 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 Slasher Movie in atmosphere.
- Forced Perspective: Used to achieve shots of characters leaving the Zygon spaceship in the final episode.
- Hand Puppet: How the Loch Ness Monster was realised.
- He Knows Too Much: The first reason why the Zygons want to kill the Doctor. Also why they kill Munro and try to kill Harry.
- Immune to Bullets: Averted. Although Broton brags about the invincibility of the Skarasen, it turns out that the Zygons themselves are not immune to Earth weaponry. Benton wounds one and the Brigadier even puts down Broton at the end.
- Impaled with Extreme Prejudice: The Zygon impersonating Harry tumbles off the haystack after lunging at Sarah and is promptly skewered with his own pitchfork.
- It Was Here, I Swear!: Sarah discovers a bookcase leading to a secret passageway in the castle. When she returns with Harry, they find the incriminating evidence removed.
- Kubrick Stare: Zygon!Harry, constantly.
- Left the Background Music On: 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.
- Malicious Misnaming: The Duke (actually Broton) deliberately calls the oil company manager Huckle "Heckle"
- Man in a Kilt:
- The Brigadier, embracing his Stewart heritage for the occasion.
- Also the Caber, the Duke's "Highland Games Champion" manservant.
- My Car Hates Me: Whilst fleeing the Skarasen, the Doctor's UNIT Land Rover breaks down.
- National Stereotypes: The very first line of the story mentions haggis. The last line is a joke about Scots being stingy. In between we have kilts, beards, bagpipes, second sight, and the Loch Ness Monster.
- N-Word Privileges: The writer of the story was Scottish.
- Non-Malicious Monster: The Skarasen is simply an animal, and peacefully returns to Loch Ness once freed from the Zygons' control.
- Obscured Special Effects: The Skarasen was obscured as much as it possibly could be due to the awful prop used, shown only in quick flashes of its scales or head closeups.
- Only Known by Their Nickname: "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.
- Organic Technology: The Zygons' spaceship. Only the interior appears organic, the exterior seems to be a metal hull.
- The Pirates Who Don't Do Anything: Averted. We actually see Sarah typing out a few words of a news dispatch, for once.
- Pretty Little Headshots: The man that is sniped down by the Zygons gets shot in the head and only has a small wound to show it.
- Put on a Bus: Harry Sullivan decides he'll take the train back to London.
- This would also be the Brigadier's last appearance for seven years until "Mawdryn Undead".
- In a real-world context, the Zygons, despite occasional dialogue references and enduring popularity with fans, would not appear in another official Doctor Who episode until "The Day of the Doctor" aired more than 38 years later.
- Roar Before Beating: The Skarasen roars almost constantly.
- Shown Their Work: The Brigadier is wearing the appropriate tartan for his clan (the hunting Stewart tartan).
- Stock Ness Monster: "THE Loch Ness Monster!"
- Tap on the Head: 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.
- Thrifty Scot: At the end of the story, after the Doctor and Sarah leave, the local duke whom they had rescued then berates the Brigadier (who had been earlier wearing a kilt) for calling himself a Scotsman yet not getting the Doctor and Sarah to give him their unused return tickets to London so he could get a refund.
- 'Tis Only a Bullet in the Brain: Harry gets shot in the head, but he survives because the bullet only grazed his skull.
- Voluntary Shapeshifting: The Zygons have the ability to transform themselves to resemble humans they've abducted.
- Weirdness CensorDuke: Tell me, I imagine the whole business caused quite a stir.Brigadier: No, the Cabinet's accepted my report and the whole affair's now completely closed.Duke: You mean it never happened.Brigadier: Well, a fifty foot monster can't swim up the Thames and attack a large building without some people noticing, but you know what politicians are like.
- What Could Possibly Go Wrong?: 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. We all know this can't possibly go awry...
- The X of Y