Night of the full moon: Munro, an oil rig communications officer calls someone named Willie to send over some haggis -- their chef doesn't know the first thing about making it. Something breaks up their call, a strange signal that gets louder and louder. Prince Charlie Rig is destroyed by something which breaks it apart.

Early morning: the Doctor with Scottish hat and scarf on halts Harry and Sarah as they march away from the woods where they left the TARDIS. Harry wears the Doctor's usual scarf; Sarah wears the Doctor's floppy hat. He orders them forward. They see a highway. Harry and Sarah follow him down the hillside, Sarah yelling, "Hang on." A hatch back car stops driven by local landowner Duke of Forgill, who asks if they want a lift. They do.

At Fox Inn, UNIT has set up an HQ, located in the village of Tullock, Scotland. Benton and a UNIT man pull a jeep up to the Inn and hear Old Angus Ferguson MacRanald playing the bagpipes. Inside the Inn, Mr. Huckle, an American and an oil company official, talks to the Brigadier. In the last month, three oil rigs in the area have been destroyed, two of them belonging to his company -- Hibernian Oil. The Brigadier wonders where the Doctor is by now and asks Benton. He looks at the Time Space Sender device. Benton notices the Doctor coming now in the car of the Duke just as Benton is asked by the Brigadier to ask Angus to stop playing the bagpipes.

The Brigadier watches the trio disembark. The Doctor asks the Brigadier what "that" is. A kilt, being worn by the Brig. Before they enter, Huckle tells the Brigadier that the Duke doesn't own the shore base. The Duke tells Huckle that if his servant -- the Caber -- catches the oil men on his land, they will be shot. The Duke's family have served the country for seven centuries and is angry that the Brigadier cannot give him more info as to why UNIT is here. Sarah, laughingly, asks the Brigadier about his kilt, "I thought you were doing a Doctor." The Brigadier says, "What an absurd idea." The Doctor wants to know why he was called back and hopes the Brigadier has a very good reason.

Barely alive, Munro washes up on the shore.

The Doctor sleeps as the Brigadier explains the rig destructions. The Doctor is laying across some chairs and angry that the Brigadier used the psionic beam to call him two hundred and seventy million miles for oil problems. He talks about the energizing of hydrogen about an alternative source of power, not having the planet's people dependant on a mineral slime. The Doctor puts his hat over his face but when the Brigadier mentions more men dying, the Doctor acquises.

At the oil company that afternoon, the Brigadier, the Doctor, Harry and Sarah consult with Huckle about the injuries of the men -- quite a few crush injuries as well as exposure and drowning. Harry will cut off to the Sickbay. Sarah goes with him to talk to the local people. The Doctor sits trance like on the sidelines. Huckle mentions the rigs are meant to be indestructible. "Yes and so was the Bismark and we all know that story," The Doctor pops up. The sea is never empty, he tells Huckle.

In the Inn, Angus tells Sarah he is the 7th son of a 7th son when Sarah mentions Angus is rumoured to have second sight. Angus comments that the Doctor looks as if he might see around a few corners himself. She sees an animal head hanging in the Inn. It was a gift given to the Inn by the Duke of Forgill just this past week. When Sarah mentions the Duke is a rather strange man, Angus gets defensive but relents when Sarah tells him that the Duke hardly spoke a word to them as he drove them to the Inn. The Duke is the McRanald -- Angus's clan chief, a different man since the oil came. All his servants left to work with the company. The Castle is a scary place, Angus tells her.

Something watches Sarah and Angus as Angus tells her about the mist and the strange moors. Superstition Sarah says. A foreigner went out on the moor and was never seen again. Sarah makes light of this -- it was in 1922. Then there was the Jameson boys -- 1870 -- went out cutting pete. Donald vanished. The older brother was found two days later, never spoke again. Ancient mysteries and evil spirits haunt the moor. Sarah is certain that evil spirits don't destroy oil rigs.

Munro rises and walks further onto the shore. Amid the dunes, the Caber spots him and is carrying a rifle. From his jeep, Harry also sees Munro and stops. Harry runs to him and as Munro tries to tell him what came out of the sea to attack the rig, the Caber shoots at them, killing Munro and wounding Harry -- a shot grazing his forehead and putting him out.

At the inn, Sarah returns as the Doctor works amid the bagpipe music. The Brigadier is on the key side, watching wreckage being brought aside. Sarah comments about the landlord having second sight and the bagpipe music stops. The Doctor tells her he was playing Flowers of the Forest, a lament for the dead. A radio probe system used for checking localized jamming is what he is working on. Sarah asks what if that gets jammed too. The Doctor just stares at her with a disapproving face. A phone call to Sarah alerts them to the fact that Harry's been shot.

Alien hands and hissing, low voices work on strange suction type controls. They order a giant serpent under the water's surface back into a closed area that they have. Huckle is talking to Number 3 Rig-the Ben Nevis rig as a communications black out occurs -- a loud sound whining.

At the oil company sickbay, the Doctor meets the Sister Lamont who watches over Harry. Harry is still in shock. The Doctor tries to reach the unconscious Harry. The Brigadier and Sarah come in and the Brigadier explains the Ben Nevis oil rig has been destroyed--40 men aboard vanished. The Doctor and the Brigadier leave; Sarah stays with Harry, but Lamont thinks she should let him rest. Lamont walks out as Sarah watches Harry, who is under sedation.

The Doctor sees a part of wreckage from the Prince Charlie in the jeep. He asks Mister Benton for some plaster of paris in the sickbay. Benton goes. Soon the Doctor has made a plaster of paris model of the holes made in the metal. In the Inn, it is complete. When Huckle asks what the Doctor is doing, the Doctor explains -- orthodontologythe study of teeth -- the cast of the teeth coming from the metal wreckage. They are watched by alien eyes who recognize the Doctor as a threat who must be destroyed.

Harry starts to wake up, his head bandaged. He starts to tell her something when Sister Lamont silently returns. Sarah asks about Munro. Harry starts to tell her more but Sarah moves to call the Doctor. Lamont sinisterly tells Harry that his worries are over now--he will be looked after. Harry tells about Munro but a strange sound is in the room with Harry who yells, "No, no!" Something comes at him.

The Doctor and Benton rush to the sick bay where Lamont tells them Sullivan has vanished...and so has Sarah. The window was open. Lamont claims to have gone to Hut 4 to find Sarah and that is when Harry vanished. The Doctor asks Mr. Benton to get some men and search. The Doctor checks out a decompression unit which is used for the divers. Lamont says she will search the dispensary. The Doctor finds Sarah in the decompression unit and enters. As Sarah explains, the thing shuts the unit and locks them inside, closing the blinds. It also starts to pump the air out.

Two Zygons bring Harry into a control centre to face the Zygon leader -- Broton, war lord of the Zygons. Centuries ago, their craft was damaged and crashed in the Loch. Their home world has been destroyed in a stellar explosion. Now they will make Earth their's. Using the suction devices, they show Harry the giant monster in the Loch and how they control it.

Sarah finds it hard to breathe. The Doctor tells her to shut up and save her breath. He looks at her worriedly.

The creature, a Skarasen, swims toward the Zygon ship, both underwater. The Zygons depend on the lactic fluid of the creature, which is grown from an embryo. It is a armoured cyborg.

The Doctor is hypnotizing Sarah, telling her, "Do not breathe." He tells her she feels nothing. He then closes her eyes and looks up, rolls his eyes, and makes a strange sound.

Fox Inn: the Brigadier calls in a UNIT corporal who is in contact with the Coast Guard. He irritates the Brigadier by continually saying, "Sir." He set up a group at a point to listen for the monster. Nerve gas comes in from a side door and puts the Brig, the Corporal, and the other men in the room to sleep.

Benton finds the Doctor and Sarah but can't get the door open right away. He uses the electronic controls to open it. Benton awakens the Doctor first. The Doctor says, "It worked, Mister Benton." Benton whispers, "What worked?" "Why are you whispering?" The Doctor tells him a trick he picked up from a Tibetan monk. The Doctor also warns him not to touch Sarah -- it could be fatal to break the spell incorrectly.

Huckle enters the Fox Inn to find the Brigadier and men sleeping. He hears the cries of the monster. He is carrying a strange signal device he found among the rig wreckage. A UNIT soldier is in the mist. The monster roars its head at him.

Later, Benton tells Sarah everyone in the village has been affected by the gas that affects the higher consciousness. The Doctor reasons that someone wanted something large to pass by the town unseen. The Doctor tells Mr. Benton to get outside on watch. Sarah laughs. "Is it possible?" The Doctor asks.

The Zygon reports to Broton about the Doctor and the female human. He wants an explanation from the one who locked them in. Broton puts on the screen and sees the Doctor awakening the Corporal. They witness Huckle giving the Doctor the triolanic signal locator. Broton wants it recovered so he has two of his guards take Harry for use. The Doctor explains it is a signal device for the thing chewing up the rigs. Broton feels the Doctor is too clever and dangerous. The Zygons tell Harry they need his body print. The Caber, the Sister Lamont, and the Duke are in devices with their heads covered by part of the device. They are not dead but their body prints allow the Zygons to use the form as their own for a time. One of the Zygons makes himself into a replica of the Caber.

With his eyes closed and head on the table at Fox Inn, the Doctor figures the device might emit some kind of primeval mating call to the monster. If one of these is clamped to the rigs...the thing would attack the rig. The Doctor tells Mr. Huckle he's done enough...what he found could be very valuable. Huckle tells them to watch themselves and leaves. The Brigadier comes around and find it impossible that he was asleep on duty, "There are times, Doctor, when you do talk absolute nonsense." The Doctor and Sarah laugh.

Benton and a UNIT man find a dead soldier in the fields along the moors. He sends the man to fetch the Doctor and the Brigadier. The trio leave and Broton thinks that since the female human is alone...it is excellent. The Doctor and Brigadier inspect the body. With a pencil in her mouth, Sarah types alone in the Inn when Harry walks in... morose and angry. He claims to have escaped, picks up the Zygon device, and tells her the Doctor told him to bring it to him. He also knocks her down when she starts to follow him. She does anyway, giving chase and calling to him. Sarah uses her head and gets a group of three UNIT soldiers to give chase also. Harry flees to a farm. Sarah suggests she and the UNIT men split up to find him. Sarah hears noises in a barn and goes inside. The alien Zygon in the form of Harry hides among the hay in the loft. Sarah climbs the ladder and goes to the top. Harry, the alien, attacks her with a pitchfork. She moves aside at the last moment and the creature falls. Sarah peers over and finds the Zygon's monster body has returned to its natural state. The real Harry stirs. Broton believes the Zygon is dead and must be dispersed when there is no synchron response from it. Sarah brings the soldiers back but the Zygon's body is dispersed via remote control.

Sarah and Harry discuss with the Brigadier the fact that they are either being watched or that one of them is a Zygon. The Doctor whispers to the Brigadier it is more likely that the aliens have a bug hidden that is watching them. The Brigadier orders Benton and his men to search for it.

Broton wants the unit with the Doctor destroyed. Broton will not tolerate argument from his underling when the underling does not want to activate the Skarasen to attack the Inn to get the device. The underling does not think they should betray their presense on this planet.

Sarah, in the Inn, is watching the device which is on a table. It moves. She calls the Doctor, who figures the device is part organic, part artefact. Machine guns will not be enough against the monster. The Doctor will draw it off while the Brigadier gets a trace and location from the activating signal. Sarah thinks the Doctor is taking an awful risk, "You don't know how fast this thing can move." The Doctor answers, "It doesn't know how fast I can move."

The Doctor takes a jeep to the moors but it stalls there and he tries to get it going again but the monster roars. The Doctor gets out of the jeep again and runs over the hills. The beast's large form follows him. The Doctor hurls over a hillside and jumps down at enormous heights. He continues to run past trees and mud, finally stopping at a cover. He puts his hand in his pocket, hearing the signal device getting louder and louder. He yells as the device starts to stick to his hand like a leech, "Filthy thing!" He starts to run some more.

The monster picks the Doctor up in a huge claw but Harry enters the Zygon control room and disrupts the control board, causing the beastie to drop him onto the grass and bushes. The Doctor rolls out of the way of its massive foot but has managed to get the device off his hand and to have it crushed under the monster's foot. When it registers as dead, the Zygons think the Doctor must be dead too. They recall the Skarasen. Two Zygons drag Harry out of the room. The Doctor watches the monster lumber off.

Under the Brigadier’s order, Benton and Jackson search the Inn for bugs which angers Angus, who says to tell the Brigadier he keeps a clean place. Benton humours him but tells him it is not that kind of bug. Benton almost finds the "eye" hidden in the animal head.

Sarah and the Brigadier are searching for the Doctor's jeep. The Doctor walks a bit and flags the pair down. The Doctor tells the Brigadier and Sarah the monster is a cyborg, half animal, half machine. The trio go to Foghill Castle and call for Sharpe. The Doctor sits in the chair of the Duke when it seems the castle is empty and imitates the Duke, when the Duke appears. The Doctor makes the Brigadier explain about something unusual in the Loch. The Duke finds they are serious.

Angus, dusting in the Inn, is watched by the moving eye of the animal stag head.

The Brigadier wants the Duke's permission to depth charge the Loch. "Doctor, are you a party to this kind of nonsense?" The Doctor answers, while also putting his hat on a suit of armour, "I'm not a party to any kind of nonsense." The Doctor suggest an underground river with a subterranean channel leading from Loch Ness to the Devil's Punchbowl. The Doctor explains that aliens are behind its control. The Brigadier takes to the Duke's reaction, "I know exactly how you feel sir. Before I joined UNIT, I was highly sceptical about these things." The Duke feels they are all utterly unhinged. He wonders what aliens would be doing with wireless. The Doctor says, "Well, it takes all sorts to make a galaxy, your grace."

Sister Lamont goes into the Inn where Angus is undoing the stag head where he has found the bug. When he turns his back, she starts to change but he sees the change. Lamont becomes a Zygon who kills Angus. Angus's scream draws Benton and some soldiers who get rifles to chase "Sister Lamont" through the woods. The eye of the stag is missing: the bug is gone. The UNIT men fire at the Zygon in the woods. Benton sends a man to contact the Brigadier and let him know they have one of the aliens trapped.

The Duke shows his bookcase with half the books about the Monster in the Loch. The Doctor uses the info to figure out and explain why the aliens have become aggressive. The call for the Brigadier comes in and the Doctor and the Brigadier leave, the Doctor leaving Sarah to hunt for books about the Devil's Punchbowl. Sarah insists the Doctor is leaving her out of things but he tells her it is important she get this info. "Why do I always get the dirty jobs," she says and blows dust off the shelf of books.

A UNIT man sees Lamont near a UNIT jeep, her arm bleeding (from having been shot at by Benton and the other UNIT men). This man doesn't know about the Zygon's ability to change form. Lamont clobbers him over the head with a rock when the man checks her arm wound. She steals the jeep and drives off. He must report this to Benton for Benton soon tells the Brigadier about it at The Inn. The Doctor surmises the Duke is one of the aliens... "Great Scott and we left Miss Smith alone in the castle," the Brigadier realises.

At Foghill Castle, Sarah asks to see Monasatic Records from the 11th Century onwards, which are in Medevil Latin. The Duke rings for the Caber. He continues to disbelieve in the monster. He asks the Caber to fetch Miss Smith the ladder. The Duke explains the Caber's nickname -- title -- a highland games champion, his real name is very Gaelic and the Duke tells her she wouldn't be able to pronounce it. The Caber puts the ladder in place for Sarah, who pulls a face at the morbid Caber (a Zygon in reality) and sticks her tongue at him after he has gone. Sarah discovers a secret door at the bookcase, which opens at another part of the other case. She finds a flashlight and enters a secret passage leading to the Zygon ship. The Duke returns to the bookroom and finds Sarah missing. Sarah gets inside the gloomy spacecraft which has upward sliding doors.

The Duke, the Sister Lamont, and the Caber, all really Zygons enter the space ship passage way via the bookcase; the Lamont alien still wounded. Sarah passes the real humans and then finds Harry in a cell but isn't sure she should let him free, asking if it is really him. "What on Earth's the matter with you old girl?" he asked. This "old girl" stuff makes Sarah believe it is him and she lets him out, pulling him the correct way after Lamont and the Caber leave the area. The Lamont alien claims it loathes the human abomination of a body.

The Doctor and the Brigadier return to the castle and find it deserted, also finding Sarah's coat. The Brigadier finds blood. The secret bookcase opens and Sarah and Harry come out, the Doctor hiding at the bookcase. The two explain but the Doctor wonders if they are aliens, "Nice to see you." The Doctor goes into the passageway but he screams. Broton and another exit -- warning them if they move, the Doctor will die. They will leave with the Doctor. The big event is yet to come, it tells them before leaving with his cohort. The Brigadier tells Sullivan they are not beaten yet.

Broton orders a radar jamming device put forth in order to block UNIT's tracking of the spacecraft. The Doctor tells Broton he has been hiding too long, "You've got to come out onto the balcony sometimes and wave a tentacle if you'll pardon the expression."

The alien craft heads south from Loch Ness and Brigadier orders no attack until further orders. Sarah tells the Brigadier the castle might reveal some clue as to where the Zygons have gone. The Brigadier drives Harry and Sarah toward the castle. There, Sarah and Harry hunt for clues. Harry feels if he doesn't find what opens the bookcase, he will go mad. Sarah tells Harry, "As long as you don't turn into a Zygon." "Oh I give up old girl..." Harry tells Sarah she won't find any scandal stuff here but Sarah finds a paper that says the Duke is Chieftain of Antlers Association, Trustee of the Golden Haggis Lucky Dip, and president of the Scottish Energy Commission.

Dynacron thrust is reduced as the Zygons land in the Brentford quarry. All signals are forbidden to avoid outside detection and they go on half power. "Half wit," the Doctor says.

Benton reports to the Brigadier that radar blackout all over the country. Harry and Sarah arrive from the castle by foot. The last known track of the ship was it heading south over Leistershire. The Brigadier will head back to London with UNIT. Something underwater is also heading south.

The Zygons are heading the Skarasen to a target.

The Doctor is looking at suction cups as Broton comes in, "Social call." The Doctor tells Broton he is not human and that he has seen better technology. Broton shows the Doctor that he can still become the body print of the Duke...it is needed to activate it every few hours otherwise the original pattern dies and cannot be used again. The Doctor claps and then asks questions. Broton tells him, "You like asking questions..." Other Zygons are on the way -- a great Zygon refugee fleet is coming to Earth at Broton's invitation. It will be many centuries before they arrive. In that time, the whole Earth must be restructured: polar ice caps must go, mean temp raised, thousands of lakes with mineral elements made...a new planet will be made using enforce human labour and Zygon technology. The Doctor tells him he is underestimating human beings. Tomorrow Broton will demonstrate his ultimate power.

UNIT HQ: Sarah waits with Benton, worrying about the Doctor. Benton tells her that if he knows the Doctor, the Doctor is working out a way to escape right now. The Brigadier returns and tells him the Navy is sending frigates from Chatham to deal with it. The Madam Prime Minister calls the Brigadier -- there is to be no public announcement: discreet but resolute.

The Doctor uses combined tentacles to link himself to another tentacles and this sabotages the diastellic circuit, nearly electrocuting himself. This gets a message to Benton and the Brigadier as to where the ship is. The Doctor has also jammed the door. The power runs through the Doctor and he falls. The Zygon guard thinks the Doctor is dead. Broton underestimated the Doctor's intelligence but feels the Doctor underestimated the power of organic crystalography. Broton must place the activator on the target. The Doctor comes to and realises he is alive. Hat in hand, he releases the still alive real Duke. Broton leaves via the escape hatch, walking out from under the legs of the giant spacecraft. The Doctor frees Sister Lamont and the Caber also and uses the sonic screwdriver to distract the Zygons from the control room. Alarms ring out and the Doctor makes the trio hide back in the alcoves. The Doctor was heating a fire sensor. Zygons come out and the Doctor orders them out to the control room. He orders the Caber to break a control tentacles to jam the door. "Right, let's see what other damage we can do," the Doctor says, wide eyed. He goes to a self destructor and leans on the wall, telling them they have about 60 seconds before the whole ship blows up.

As the jeeps of UNIT arrive, the alarms grow louder and louder, causing the men and Sarah to cover their ears. Sarah yells, "Doctor!"

"Everyone get down!" The Doctor yells as he, the Sister, The Caber, and the Duke come running. They hit face to sand as the Zygon ship blows to bits. "Was that bang big enough for you, Brigadier?" The Brigadier tells them that the PM was going to an energy conference near the Thames--Stanbridge House, London. The Duke tells them it is the First International Energy Conference.

At the tower, Broton, as the Duke, arrives, late it would seem. He has the monster luring device. Benton tells the Brigadier’s men that the monster will be at the House in four minutes. Harry, Sarah, the Doctor, and the Brigadier search the building. Sarah and the Doctor take the cellar -- the lights are dim. "Not much light," the Doctor tells her. "Maybe somebody should tell the energy conference," she quips. As they descends the steps, the Doctor tells her to stay at the foot of them. He hears Broton who wants to make certain the Doctor is dead. The Doctor orders Sarah to get the Brigadier but she waits.

Broton, as his Zygon form, backs up toward Sarah. Her scream distracts him but the Doctor makes for the steps too. Broton grabs the Doctor from behind and begins to strangle him from behind. Sarah runs for the Brig, who arrives with men. Thorpe, a UNIT man, enters from the other side and is choked to death by Broton. The Brigadier and Jackson shoot Broton dead. The device is in the room, "I can almost feel it." Broton must have placed it in his pocket...the Doctor's pocket. The Brigadier and men go to evacuate the building.

The Monster comes up out of the Thames. People scream, cars beep horns, and there is a stir. The Doctor runs outside on a ledge, where he tosses the device into the monster's mouth, "Fetch it." Harry and Sarah rush out after the Doctor, just in time to see the monster descending back into the Thames.

Later, in the woods, the Brig, in a kilt, tells the Duke that a 50 foot monster cannot swim upstream without some people noticing. The Duke tells them he thought he saw a sight of the monster but the sun was in his eyes. The Duke asks where this contraption of the Doctor's is. "My contraption, your grace, is this way... I think."

He can be back in London five minutes ago. Sarah says, "Just a minute, Doctor. I thought you couldn't do that." "Of course, I can," the Doctor said.