Something is destroying oil rigs off the coast of Scotland. UNIT investigates!
On an oil rig off the coast of Scotland, a strange, high-pitched beeping sound echoes through the structure. It begins to break apart, collapsing into the North Sea.
The Fourth Doctor, Harry and Sarah hitch a ride with a motorist 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 rigs have been destroyed in the last month, two of them belonging to Hibernian. The Brigadier assures Huckle that UNIT is also concerned, especially about the loss of life.
The Doctor, Harry and Sarah arrive at the inn. It turns out that their ride is the Duke of Forgill, a local landowner. The Duke is angry about Huckle’s men trespassing on his land. Huckle assures him that he has warned his employees about this, and any who are caught will be dismissed without question. However, the Duke warns that any of Huckle’s men caught on his land will be shot. He also wants to know what UNIT are doing here, but the Brigadier says that it is a sensitive matter. Though not pleased, the Duke leaves.
On the shoreline, a survivor of the destroyed rig washes up, barely alive. The Brigadier brings the Doctor, Harry and Sarah to Hibernian Oil, where Huckle briefs them on the injuries of the rigs’ crew. Harry decides to examine the injuries at the sickbay, while Sarah goes to talk to some of the locals. Huckle cannot understand it: the rigs were designed to be unsinkable and the seas were calm and empty before the incidents. The Doctor darkly notes that the sea is never empty.
Back at the inn, Sarah talks to Angus, the landlord. She 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 gone to work for them, leaving Forgill Castle cold and empty. He tells Sarah about Tulloch Moor and people disappearing over the centuries when the mist comes down. Sarah is sceptical — evil spirits do not destroy oil rigs. All this while, something alien watches Angus and her on a monitor screen.
The Duke’s man, the Caber, sees the rig survivor, Munro, stagger from the surf. Harry is driving by and and goes to Munro as he collapses. 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. Harry falls, unconscious. 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 strange, organic controls. A high-pitched beeping summons a creature from the depths. It heads towards the Ben Nevis rig. The rig’s communications with Huckle are jammed.
The Doctor and Sarah visit Harry, who is sedated and being tended by Sister Lamont, the nurse. The Brigadier arrives to tell him of the Ben Nevis rig’s destruction. Sarah stays with Harry while the Doctor goes with the Brigadier. Outside, the Doctor spots wreckage from the rig with odd holes in it. He asks Sergeant Benton for plaster of Paris and makes a mould of the holes, revealing that something with very large teeth bit the rig. The watching aliens decide that the Doctor knows too much and must be destroyed.
Harry awakens. Sarah goes to tell the Doctor while Sister Lamont tells Harry he will be very well looked after. A high-pitched beeping echoes through the room and Harry looks up in fear. As Sarah calls the Doctor, a suction-tipped hand clamps down on her shoulder. She turns as she is grabbed by a large, orange biped creature.