A breeze blows across the sand dunes beneath a white sky as the TARDIS materialises on a hillside above. The Doctor and Amy burst out. She complains about the cold but the Doctor ignores her and storms up a hill. He pretends that they are on a planet Orkadia but Amy recognises that they are on the Orkneys. They see an electricity pylon, half built, across the valley. A crowd is gathered at the foot of the pylon, jeering the workmen who are erecting it. The Doctor decides to investigate and Amy runs behind while he tells her that they are about fifteen years into her future.

The protestors' placards show their hatred of the pylons. There are about thirty of them and ten security guards facing them. The Doctor asks what the protesters want. The crowd of people wonders whether he is from Astra-gen or on their side. They are sick of the company vandalizing their islands, building illegally on the islands. They are threatening to bring the pylon down and surge forward. The Doctor tries to stop them but the Doctor steps in their way and is accidentally knocked out by a placard. The security men wade in and there is a furious fight between the two sides. The two men working on the pylon carry on, oblivious to the commotion. One man assaults the pylon base with a sledgehammer. No sooner than he hits the pylon and he vanishes into the ground. A woman screams that there is a crack in the floor. More people plunge down into it.

Amy sees the Doctor helping a large man, Thinwood, to his feet. He is on the other side from the crack in the floor. He tells Amy to get everyone away. An Astra-gen van speeds up to them and the Doctor and Thinwood are bundled into the back by the security guards. The asphalt road is bubbling as though there is tremendous heat beneath. The screams of those below are choked away. The van roars away. Silence settles other than the sobs of the protestors. A man bellows up at the workers who are still at their task. He can't believe that they are callously continuing their job. A frightened girl, Fay, tries to call police or ambulance help but there is no signal for her phone. Amy says they will try again from higher up and asks how far it is to the Astra-gen offices. Fay says that their new power station is a few miles away.

They look into the absolute dark of the split. There is no sound from below.

Thinwood complains to the security guards in the van about his treatment. He says that Lizzie knew that there would be 'Southern' protestors like the Doctor. Lizzie, it turns out, is the leader of the protest but is currently ill with Duck Flu. The Doctor surmises that the guards are in a trance like the men on the pylon. He calls the split in the ground 'unnatural'. Thinwood says that the company is building power plants without government permission. The Doctor wonders who needs all of this extra power. He lifts the visor of one of the guards to reveal the face of a sick old man. Another guard in the van collapses to the floor. The Doctor takes off his helmet to show another very sick old man, barely alive. The photograph on his Astra-gen ID card shows a man in his forties, half the age of the man they are looking at. The other guards in the van are still and silent.

The protestors make their way back to a minibus that brought some of them. Fay drives them away from the site of the incident. Amy looks back and sees that the split in the ground has healed over and the road that ran beside the pylon has widened to run underneath it. Fay says she is going straight to the hospital but Amy asks to go to the power station.

Fay stamps on the brakes as a metal lattice drops into the road before them. It is as if one of the pylons is walking, or at least trying to tread on them. It stomps towards them. Fay accelerates away, swerves past a car coming the other way and onto the grass. The road bubbles and swells and covers over the other car, leaving its own place to do so. Fay says that they must get to the nearest town.

The van halts. Thinwood says that the guards are beyond help. They hear gates clang open. Through the darkened windows the Doctor can see several pylons close together. He uses his sonic screwdriver to open the door. They are in the approach to spacious grounds with a sharp white building in the distance. As Thinwood steps out of the van the driver's window opens and reveals an aged woman who tells him to get back in because 'they' will want to speak to him. He recognises her as Sally Preston, a woman who should be in her twenties rather than her sixties. Unnerved, Thinwood tries to walk away but one of the nearby pylons lowers its arm, snapping cables that strike the tarmac beside him like lashes. He scrambles back to the Doctor who is fascinated: are they robots, bosses or henchmen. The pylon steps closer to them, its cables lashing the air. Thinwood and the Doctor enter the grounds of Astra-gen followed by the pylon.

Fay drives on over scarred roads. Pylons stride across the horizon, trailing cables behind them. They reach the roundabout outside Kirkwall. Amy is worried that it might break up but before she can speak her fears a crowd of people flee across the road pursued by a pylon that crashes through buildings. It has a cloud of yellow energy around it. It lashes with cables at the people fleeing before it, killing several. Fay accelerates beneath the pylon. They enter the ruins of Kirkwall. Some survivors cling in the wreckage. They pull up outside the cathedral, now smashed into two halves. Amy wonders if this is mindless violence or part of some plan. Fay leaves the driver's seat and asks an Asian guy to use the bus to take injured people up to the hospital. She and Amy are going to Astra-gen.

Thinwood and the Doctor run towards the building. The Doctor uses his screwdriver to turn on the music in two parked cars. Instantly the tarmac around them splutters into heat and the speakers in the cars spark and are silenced. The Doctor wonders what the link is between metal and tarmac. He uses the screwdriver to start every car in the car park and then runs with his new friend to a back door. Using the guard's pass key that he took earlier he gains entrance. Behind them the car park is suddenly silenced.

Fay drives an abandoned motorbike across the countryside with Amy clinging on behind. A mist is rising. The only pylons are in the distance. Every road they see is cracked and broken. They make for the top of a hill and look down onto the Astra-gen building. Pylons move around in the distance.

Thinwood follows the Doctor through the dimly lit corridors of the top floor. They peer into the managing director's office. Patterns of light swirl over the monitor of a computer. The Doctor says it is extra-terrestrial and this is an interface transferring energy from somewhere to somewhere else. He wonders what the power is that is spilling from the pylons and what is generating it. With his screwdriver he calls up a map on the screen. Thinwood tells him it is Orkney. A ring of red dots cover the map but Thinwood says that all of the roads are in the wrong place and that the new roads look like a circuit soldered by pylons. The Doctor tells him that it is clearly the work of an alien intelligence.

They go down to the boardroom. As the Doctor reaches for the door handle it grabs him and pulls him inside. Thinwood follows him in. the managing director, Anna Price, and her six board members, now hideously aged, are around the table. They are connected by cables to a steel cube at the back of the room. A chair comes round behind them. The Doctor demands to know who is behind this. Price whispers that all he needed to know was left on the view screen in the office. They have been brought here as representatives of their groups. The Doctor demands that the aliens leave Anna alone and reveal themselves. A dark, slimy shape in armour manifests itself. It apologises, saying that to survive its people must steal and slaughter.

Amy and Fay ride down to the security van. Amy tries to remove the helmet from one of the guards but his head snaps off in her hands. The driver, too, is dead and emaciated.

The Doctor seems impressed by the bizarre shape hovering before him. The creature says that it has come from its ship which is in low orbit around Earth. It is of the Refined Grade Caskelliac but the Doctor says he detects PAH rather than DNA. This is a molecule found in oil, tar and coal. This explains how the creature can control tarmac. He sees that the creature is wearing a steel exoskeleton. The creature demands to speak in order to explain itself: it was created as a weapon of war by an ancient race. Its task is to pollute and destroy worlds. It, and its fellows, are programmed by nanocreatures that repair and maintain them and their shells. Thinwood asks why they are making a circuit. The creature says it is using the planets resources and is using the circuit to broadcast an electro-chemical energy field to poison the air and oceans so that they can feed on all life on the planet.

The makers of the creatures were destroyed in their war but their weapon drifts on restlessly. The creature says that the Doctor has helped by listening. Anna Price's body keels over an explodes into dust. The creature begins to fade away as two more board members explode. A cable snakes out to wind itself around the Doctor's wrist. Amy rushes in as the Doctor collapses to his knees telling her that the steel box is alien and draining his energy. Then the Astra-gen van bursts through the wall and the power cables are released. The Doctor looks at the empty driver's seat as Fay enters.

The steel box straightens out its dents so the four of them run outside. The air seems to be thickening. The Doctor says that the intention is to suffocate the population. He wonders why they chose Orkney as the platform for their invasion. Amy suggests that the aliens want to be somewhere quiet, hence their turning off the music in the cars. They are also easily distracted and not up to full power. He uses the screwdriver to blow up a generator in a shed as a further diversion. He then runs off to the helipad where a helicopter is waiting. He says he has an unwieldy plan but needs their help.

He lifts off noisily in the helicopter while the other three make their way back to Kirkwall. He speeds towards the coast. The metal housings in the cockpit begin to melt and run. He uses his screwdriver to hold back the nano-machines briefly.

The other three jolt back to town on the motorbike. The sky is darkening and the air is hot. It stinks of tar. It is difficult for them to breathe. Their plan is to cause a noise to attract the nano-machines in order to give the Doctor the chance to act.

The Doctor aims for the most vulnerable spot, knowing it will be protected. A pylon shoots bolts of metal at him, shattering one of the windows. Other pylon wires strain towards him and try to bring down the helicopter but he dodges past until the lever jams. He jerks it free and turns towards the heart of the ring of steel. The air burns in his throat. He sees the new-built pylon, bigger than all the rest. The two workmen lie dead beneath it. The rotors fail and he knows he will be held stationary in the air.

Amy leaps from boat to boat in the harbour. Thinwood looks up at the bell tower of the cathedral and improvises a music system while Fay coerces two hundred people to emerge from the rubble. Amy returns from the harbour dragging an extension lead that she has attached to the battery in one of the boats. Thinwood tells the crowd that the pylons are trying to kill them but their only defence is to chant loudly. As the crowd begins the bells chime and the boats sound their foghorns. The cacophony intensifies. A pylon looms out of the night towards them. The crowd scream in terror and the pylon buckles and topples into the waters.

The air thins around the helicopter as the nano-machines stream towards Kirkwall. He steers slowly towards the pylon. The air thins and the helicopter crashes into the pylon. It bursts into flames as the Doctor leaps out. The energy circuit is broken and the air clears. He lands on grass as the tarmac bubbles and hardens. The alien spaceship appears briefly, and then dissipates. He sits up, realising that the aliens have gone.