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.
On the shoreline, a survivor of the destroyed rig washes up. The Brigadier brings the Doctor, Harry and Sarah to Hibernian Oil, where Huckle briefs them on the injuries of the rigs' crew. Harry examines the injuries at the Sickbay, while Sarah goes to talk to the locals.
At the Inn, Sarah talks to Angus, the landlord. Sarah admires a stuffed stag's head above the fireplace, a gift from the Duke just this past week. Angus observes that the Duke has not been himself since the oil companies came. His servants have left to work for them. Something alien watches her and Angus on a monitor screen. Angus tells Sarah about Tulloch Moor and how people have disappeared over the centuries when the mist came down.
The rig survivor, Munro, is spotted by the Duke's man, the Caber. Harry is driving by and sees Munro too. Before Munro can tell Harry about what smashed the rig, the Caber fires a rifle, killing Munro and creasing Harry's forehead with a second shot. Back at the Inn, the Doctor is working on a radio probe to check for localised jamming when a call comes in informing them about Harry.
Alien hands manipulate organic controls. A high pitched beeping summons a creature from the depths that heads towards the Ben Nevis rig. The rig's communications with Huckle are jammed by the beeping.
The Doctor and Sarah visit Harry, who is sedated and being looked after by Sister Lamont, the nurse. Outside, the Doctor spots a piece of wreckage from the earlier rig with some odd holes in it. The Doctor asks Sergeant Benton for some plaster of Paris, and makes a mould of the holes, revealing very large teeth. The aliens, watching, decide that the Doctor knows too much and must be destroyed.
Harry awakens. Sarah goes to inform the Doctor, while Sister Lamont tells Harry that he will be looked after. A high-pitched beeping echoes through the room. As Sarah calls the Doctor, a suction-tipped hand clamps down on her shoulder. She turns and is grabbed by a large, orange biped: a Zygon.
The Doctor rushes over to the Sickbay. Sister Lamont tells them she found Sarah gone and Harry's bed empty. The Doctor finds Sarah hiding in a decompression chamber. As Sarah starts telling the Doctor about what happened, a Zygon locks them in and remove the air from the chamber. The Doctor hypnotises Sarah into not needing to breathe. He closes her eyes and then, breathing in loudly, places himself in a similar trance.
Harry is brought to the Zygon ship deep underwater, where he meets their warlord, Broton. Harry is told that centuries ago, their spaceship was damaged and they landed on Earth. They were waiting for rescue when their world had been destroyed in a stellar explosion. They intend to claim Earth for their own, using the sea monster under their command, an armoured cyborg of great power. Broton explains that the Zygons depend on the lactic fluid of the Skarasen, and Harry realises that if the monster is destroyed, the Zygons will die.
The Brigadier is briefing one of his officers at the Inn when gas floods the room, knocking them all out. Meanwhile, Benton has found the Doctor and Sarah. He manages to open the pressure door. The Doctor tells Benton that the trance was a trick he picked up from a Tibetan monk.
Benton, Sarah and the Doctor discover that the entire village has been drugged by some kind of nerve gas, but they soon revive. The Doctor deduces this was done so that something could get around unseen. The Zygons watch Huckle give the Doctor a device he found in the wreckage: the signal device that summons the Skarasen. Harry is taken to another room in the ship, where he sees other humans hooked up to booths, providing the Zygons with "body prints" that they use to assume human form.