On a North Sea oil rig, the radio operator Munro's transmission is interrupted by a high-pitched beeping noise. The lights go out, the rig mountings shatter, and the structure falls into the sea.
The Doctor (dressed for Scotland) stalks through the woods with Sarah Jane and Harry in tow. They eventually find the road and flag down a passing motorist, getting a lift to the nearby village where the Brigadier has set up a base of operations at the inn.
The Brigadier is talking with Huckle, a representative of the oil company that owns two of the three rigs that have been destroyed in the last month. The landlord is playing his bagpipes extra loud to annoy the soldiers who have taken over his inn.
The Doctor finally arrives and the Brigadier asks Huckle to identify the driver, the Duke of Forgill. He comes in with them and wants to complain to Huckle about his men trespassing on his lands. The Duke warns any men on his land will be shot and then demands answers from the Brigadier, who isn't forthcoming.
Nearby, Munro washes up on shore.
The Doctor is less than impressed with what he considers the Brigadier's abuse of the space-time telegraph. The Brigadier points out that more men will die and the Doctor agrees to check it out. They get the medical reports from Huckle and Harry notes there is an unusual number of crushing injuries. Harry goes to check out the bodies. Huckle claims the rigs were unsinkable and the seas were calm, but the Doctor is unconvinced.
Sarah Jane talks with Angus, the innkeeper, and wonders why the Duke was so quiet. Angus admits the Duke has changed since the oil companies arrived. They are unaware that some alien is watching them through a spy-eye planted in a stuffed elk head, a gift of the Duke. Angus claims the local Tulloch Moor is haunted and several people have disappeared or been driven insane over the last century.
Harry is driving back when he finds Munro. He talks to the man who is dying but warns that something came out of the sea. Before he can say anything else, the Caber, a man working for Forgill, spots them and opens fire with a high-powered rifle. His first shot kills the operator and his second drops Harry with a head wound.
Sarah Jane talks to the Doctor who is working on a radio unit to check for localized jamming. They get a call from the hospital, telling them Harry's been shot. The aliens work at their controls, sending some vast aquatic beast through the ocean. Huckle is talking with the newest rig when they're interrupted by the same high-pitched signal.
The doctor tries to interrogate Harry without results but realizes he will recover. The Brigadier arrives to reveal that another rig has been destroyed. Sarah Jane stays with Harry while the nurse, Sister Lamont, advises her that he needs his rest. As the Doctor leaves, he notes a piece of wreckage and makes a plaster-of-Paris mold of the holes…which are the marks of giant teeth. The alien leader, looking on, realizes the Doctor knows too much and determines he must be destroyed.
Harry wakes up and tells Sarah Jane about Munro, and she goes to tell the Doctor. Sister Lamont looks on as Harry talks about what he saw, but she begins to…change, and Harry draws back in horror. The Doctor tells Sarah he'll be right over, but she's interrupted when a giant orange biped creature with suckers comes up from behind and grabs her.