The TARDIS makes an uncomfortable landing, as if it were drawn down by some great magnetic force. Because of this, the Doctor has no idea where, or when, he and Sarah have arrived. The TARDIS had materialised on an isolated, deserted beach. As the Doctor and Sarah emerge to investigate their surroundings, the feel not only the bitter cold, but also that they are being watched...

In reply to the Doctor's query of what month it is, Sarah says that she thinks it is February. From this, the position of the stars, and the wind direction, the Doctor concludes that they just might be in England.

Sarah notices a notice board along the beach. The Doctor shines his torch on it and they read it. "Ministry of Defence. It is dangerous to swim from this beach at any time. Keep out." To this the Doctor remarks that the tide so shallow, there's barely enough water to paddle. Sarah is puzzled, then, at the reasoning of the notice board.

The Doctor notices lights in the distance on the other side of the bay, to far to reach on foot. "Especially in this mud," Sarah replies as she discovers something which appears to be seaweed. Sarah remarks on its resemblance to a baby octopus, with an eye and tentacles. On closer examination, the Doctor discover that it is comprised of some metal substance.

Not wanting to indulge the Doctor's curiosity, Sarah implores him that they return to the TARDIS. The Doctor, ignoring the request, bids her to listen. As she cannot hear anything, she accuses the Doctor of trying to frighten her. Still, the Doctor is insistent that she listen.

It is soon apparent they they are not, in fact, alone. Something is stalking them in the black night, approaching closer and closer. As the moon emerges from behind black clouds, they catch a brief glimpse of the creature before it attacks. Not waiting to see what their assailant is, the Doctor and Sarah run away down the beach, the creature in pursuit.

They elude the beast and later discover that they have arrived in the British Isles in the present day. They learn that a government underwater expedition has vanished from the estuary bed. This expedition was searching for a meteorite that had fallen into the sea, causing flood damage up the River Thames. Professor Emmison, an eminent astronomer, informs them that this is the third expedition to vanish without a trace, and that the river is beginning to show signs of radioactive contamination. Against the professor's most stern warnings, the Doctor goes to investigate the estuary bed.

The Doctor makes his way down into the murky depths of the estuary. Here there are no signs of the life that would normally make up a healthy ecosystem. The Doctor's explorations lead him to the gigantic cavern formed by the meteorite impact. Exploring this, the Doctor discovers a tunnel hollowed out into the seabed. As he travels along this tunnel, he realises that water is growing colder and darker, a sign that the tunnel is plunging further down. The tunnel grows narrower and narrower until it takes a turn to the right. Something unseen brushes against the Doctor's body. It begins to capture the Doctor in a vice-like grip, starting with his ankle.

It soon entwines his whole body, squeezing the life out of him. He drops his sea lamp, and by the filtering light, he sees the creature, a great living metallic weed. It continues to grip and crush the Doctor, preparing to feed, engulfing the Doctor. But then suddenly it stopped and disappeared into the darkness.

The Doctor regains his composure and strength, along with his sea lamp. The sene it illuminates is a grim one. Bobbing rhythmically with the movement of the water, dismember human bones and skeletons beg for the help that never came.

The Doctor presses on deeper into the tunnel. Gradually a fluorescent glow begins to illuminate the tunnel. He has reached the resting place. Not of a meteorite, however, for it is the reaing place a vast cylindrical, burnt out spacecraft.

The Doctor, on closer examination, notices that the surface is covered with claw marks resulting from a life or death struggle to escape. Suddenly, the air in his diving helmet grows thin. Pressure builds in his throat, as if he were being strangled.

He hears the chilling sound he heard on the beach: a heart beat. He turns his lamp to illuminate an approaching creature. One he has seen before. His worst fears realised, he escapes the tunnel thinking of the threat he had seen so long ago..

Sarah helps the Doctor out of the water and he warns her of the enemy they now face: Pescatons. He describes them as cunning, ravenous creatures. Half-human, half-fish creatures originating from the Carcariday, the deep sea water creature like the shark. They are the most ferocious species the Doctor has ever met, attacking and feeding on anything that comes in their path. In response to Sarah's question to their appearance, the Doctor describes them as having the head and body with the shape and texture of any shark species. Teeth sharp as nails, fins like claws, they tower over any of their human victims. They move slowly on two webbed feet. Sarah is sceptical at the concept of fish that can build spacecraft and travel to other planets. The Doctor informs her that the Pescatons have developed technology far superior to any on Earth. Sarah asks where they come from. The Doctor replies that they come from Pesca, once a planet covered with vast oceans, now a vast wilderness. He confirms that he has visited it before.

Experts dismiss the Doctor's findings as pure fantasy at best and lies at worst. Despite their denial of its existence, the Pescatons comes out of the river into the heart of London, striking fear and terror into the inhabitants. Night falls. As the Londoners wait for the next onslaught from the Pescaton, it retreats into a canal, visibly weakened in its search for salt water. It emerges later in the grounds of the London Zoo, its strength declining further, like any fish out of water. Time is running out for the Pescaton.

The creature slumps to the ground, its heart beat fading away to silence. Figures emerge from the shadows against the Doctor's warnings to examine the beast. They announce that the creature is dead and more gather around to satisfy their curiosity. The group gathered around the creature grows until it literally engulfs it.

Without warning, the creature rears up, striking out with unrelenting fury at the crowd around it. The crowd disperses quickly, but not quickly enough to escape the teeth and claws of the ferocious Pescaton. Its newfound strength rapidly decreasing, it heads towards its last and only hope for survival: the aquarium house.

Here in the tanks it could find the salt water and food it required. The Doctor advances cautiously into the aquarium hall to stop this Pescaton threat. He stops and listens. Silence. The creature is lurking somewhere, unseen. He wonders if it could have expired. Then the heart beat of the beast. Out of the shadows at the other end of the hall, the emerald eyes of the creature emerge. With superhuman strength, it smashes a nearby tank, sending water and marine life onto the floor. The Doctor tries to get away before the other tanks fall on him. And then the Pescaton collapses to the floor, dead. Frozen with fear, the Doctor stands and watches the creature, its flesh disintegrating before his eyes. All that is left is the vertebra of a large fish.

Londoners return to their daily life, relieved of the Pescaton threat. That night, however, meteorites fall one by one into the murky River Thames...

Reports flood in from every corner of the globe of meteorite landings. Earth is on the brink of hostile colonisation by the Pescatons.

The Doctor casts his mind back to his first encounter with this alien menace...

For thousands of years, Pesca, in the constellation Pices, had been covered with oceans. But now the planet's orbit brought it closer to the sun, drying up the seas, destroying all marine life.

The TARDIS lands and the Doctor steps out onto hard, baked soil. The desert-like surroundings show no sign of life, neither plant or animal. The Doctor discovers marks of marine life on what had once been a seashore. As the Doctor crosses a dry riverbed, the ground gives way and he is cast into an endless chasm.

As the Doctor comes to, eyes still closed, he notices he is completely paralysed, seemingly stretched out on the floor of some vast cave. He listens to the relaxing sound of lapping water in the underground reservoirs of this planet until he hears a heartbeat. His eyes pop open, revealing that he is surrounded by creatures pulsating to the rhythm of their own heartbeats. Above them all is a bigger creature with an oval-shaped head too big for its metallic scale-covered body. The Doctor becomes transfixed by its emerald eyes.

The creature speaks, informing the Doctor that they have been waiting for him. This is Zor, leader of all Pescatons, who brought the Doctor to their planet to help them find a new planet, as theirs is in danger of being destroyed. The Doctor refuses and Zor locks with him in mental battle. In this, the Doctor learns that Zor is the nervecenter of the entire Pescaton race. The Doctor wins the battle and escapes to the surface. As the TARDIS takes off, he has a feeling that he would meet up with this evil race once again...

After the first wave of landings, London settles into an uneasy calm. All the streets are deserted, leaving a constable to patrol his empty beat. No-one is out to feed the pigeons, Snowflakes begin to fall and settle in Trafalgar Square. The city is waiting.

A river tug makes its way to the Great Pool of London. The attack is swift. A Pescaton rears up out of the water and rips into the boat. The advance guard emerges from the river, all along from the sea into the city of London itself. The streets are now filled with chaos, people everywhere in the path of the Pescatons. All the authorities' attempts to stop them come to naught.

Amidst this chaos, the Doctor and Sarah travel down a back street. Sarah discovers an abandoned baby trying to return to its mother near a shop door. The Doctor tries to talk to and is puzzled when it doesn't reply. Sarah inform him that it cannot. She wonders why the Army, with all its sophisticated weaponry, can't stop this alien force. The Doctor informs her that all their weapons are useless: Pescaton skin is like steel. Sarah begins to get agitated at the fact that London is being destroyed, and yet there is nothing that can be done. The Doctor asks her for her suggestions on how this could have been avoided. She replies that everyone should have been prepared, as there are many hostile race that could attempt this very thing.

A Pescaton appears at the end of the street. The Doctor bids Sarah to go with the baby, but Sarah will not leave him. The creature moves into attack, apparently towards Sarah and the baby. The Doctor tries to gain its attention by throwing various discarded items, but to no avail. Finally he tries song and dance. His rendition of "Hello Dolly" turns the creature's attention to him. The Doctor manages to slip away and meets up with Sarah later.

But something puzzles the Doctor: without their nervecenter, Zor, the Pescatons' power is useless. Unless Zor himself is on Earth. If so, he must be found with all due speed. Only then could the invasion be stopped.

The first wave of attacks cease. The Pescatons retreat to the Thames. In an attempt to do this, one creature gets trapped in the Underground. Sarah and the Doctor decide to go after it.

As they make their way along the tunnel, the Doctor listens to the walls for vibrations. He plays his piccolo, as he always does when he is nervous. The creature appears behind the Doctor, and his instinct is to flee. But Sarah has discovered something: the creature is trembling. Ignoring the Doctor's plea to flee, Sarah bids him to play again. He does and the creature becomes more agitated and moves off down the tunnel.

Quite unwittingly, they have found the one weakness of the Pescatons: high pitched noise. The hunt was on for Zor, He is found in a sewer underneath the city streets.

With the help of Professor Emerson, the Doctor sets up high frequency sound equipment around the walls of a sewer cavity. The Doctor waits for Zor.

The wait is not long. Zor moves close to the cavity, but just short of the activation barrier. Zor stares into the Doctors eyes, draining him of power. In the process of doing thing, however, Zor moves into the activation barrier, setting off a high-pitched whine that cut right into his brain. Zor's heart beats faster and faster until it stops. Zor had disintegrated before the Doctor's eyes. So too, had the entire Pescaton race. The planet of Pesca, too, is destroyed by its own sun.