During a routine days work in a UNIT Tracking Station the radar technician is suddenly alerted by the sight of a swarm of meteorites on his screen, flying in formation they approach the Earth. Responding to his Officer, they both watch as the alien matter appears to slow down. It is almost as if the mystery objects are going to land.

In another part of the country, Oxley Woods, a poacher, Sam Seeley dives for cover as several "thunderballs" crash around him. As the quiet of the wood returns so does Sam Seeley's courage and he investigates. Probing the ground with a stick he finds a "thunderball". Of an irregular shape the artefact emits a warbling sound in time with a pulsing glow. Touching it Sam recoils as his "thunderball" is still red hot. He reburies it intending to collect it later. In another part of the woods the wild life is disturbed yet again, this time by the sound of some alien engines. Slowly the TARDIS materialises and as the doors open an unfamiliar, shabby Doctor falls to the ground.

Later that day a black Ford Executive speeds through London carrying scientist Liz Shaw to a secret rendezvous. She has received a communiqué from Brigadier Lethbridge-Stewart requesting her presence at a meeting regarding the post of Scientific Adviser. Once finished explaining, about UNIT and the job the Brigadier is surprised by Liz Shaw's scepticism and reluctance to help. Only when mentioning the meteorites and that another shower had "landed" in exactly the same spot some months earlier, did she become interested.

Their conversation is interrupted by a phone call from Captain Munro. His troops have cordoned off Oxley Woods and searching for the meteorites have come across an unconscious man beside a Police Box. The words Police Box spur the Brigadier into action. Could this stranger be the Doctor? A guard is placed around the TARDIS and he and Miss Shaw travel down to Ashbridge Conage Hospital to have a look at the mystery man.

Meanwhile, at the hospital our stranger is causing much confusion. The Doctor in charge, Henderson, studies some X-rays which show the injured man to have two hearts. Going to the phone to check, he in turn receives an irate call from a Dr Lomax demanding to know what Henderson is playing at sending him non human blood for cross matching. Dr Henderson is baffled. He took the blood sample himself. One of the hospital porters, Mullings, overhears the telephone conversation and sells his story to the Daily Chronicle.

By now Sam Seeley has returned for his "prize". Digging it up he puts it in a sack and sets out into avoiding the searching troops.

The Brigadier and Miss Shaw arrive at the hospital only to find a large press reception asking about a man from space. Furious, the Brigadier pushes past them to meet Dr. Henderson. Shown into the room the Brigadier is disappointed that the man on the bed is not the Doctor. Just then the patient stirs and smiles up at the Brigadier, greeting an old friend. Seeing the puzzled look the mystery man borrows Liz's pocket mirror and examines his face. Looking worried he slips back into unconsciousness. Baffled the Brigadier orders him to be moved to UNIT HQ as soon as possible.

Out in the main entrance of the hospital the reporters see the Brigadier's car speed off. Realising something is afoot they rush to the phone where one man is just standing, staring straight ahead. As the phone box door is opened, without a word he turns and leaves. Whatever he was doing, he certainly was not using the phone.

Meanwhile, our poacher is getting careless and is caught by the two soldiers guarding the TARDIS. They ask if he has seen any meteorites. Sam acts the fool. They let him go warning him to keep away from the area.

Back in the hospital the celebrity patient suddenly wakes, demanding his shoes. The nurse gives them to him and goes oft to fetch Dr Henderson. Seconds later they return and the figure on the bed falls immediately into a deep sleep clutching the shoes desperately to his chest. Worried that they may possibly have a mental patient on their hands Dr Henderson and the nurse go off to fetch some equipment. As soon as they are gone the stranger wakes and shaking the right shoe, a key drops out. Holding this tightly he slips into unconsciousness again. This time smiling.

A short while later Dr Henderson returns and waiting for the nurse sits at the desk making notes. Silently two figures creep up behind him. One hits Dr Henderson on the back of the neck, knocking him out cold. Then the two grab the patient, put him into a wheelchair and put a strip of elastoplast over his mouth. Hurriedly they push him out of the hospital to a waiting ambulance. As they push the wheelchair into the ambulance two strangers suddenly twist the wheelchair around and dash off down the drive. The kidnappers quickly board the ambulance which gives chase and is driven by the man found earlier, hogging the phone box.

Just at this moment Captain Munro finds the dazed Dr Henderson and rushes outside just in time to see the speedily retreating ambulance.

He orders his men to shoot at the tyres but they miss.

Only a superficial wound, the private is nevertheless severely reprimanded. Dr Henderson takes his charge back to the hospital and informs the Brigadier of events. Later while doing an ECG scan it is revealed that there is very little brain activity in the patient. Dr Henderson believes this to be self induced and nothing to do with the bullet wound. The precious key is prised from the stranger's hand and given to the Brigadier. He wonders. Could this man be the Doctor? First the Police Box, then the key and the stranger's efforts to regain the TARDIS.

With the TARDIS moved to UNIT HQ the guard on the hospital is doubled. The searching troopers find shattered pieces of meteorite which the Brigadier takes with him for Miss Shaw to analyse. Just as he is leaving Captain Munro shows the Brigadier a press photo which contains a shot of the leader of the kidnappers. He tells Captain Munro to run a check on the suspect.

Returning from a business trip in the States Ransome goes straight to his factory, AutoPlastics Ltd, to find that all the staff have been replaced and a notice on his workshop door saying "Out of Bounds"! Going straight to the Managing Director, Hibbert, he demands to know what is going on and why he has been dismissed. Hibbert rather feebly explains that the firm has gone into partnership and that new policies are being implemented. Ransome protests but is asked sharply to leave. Shocked that his old friend could treat him like this Ransome complies. As he leaves he is watched by the new partner, Channing. He was the leader of the kidnappers!

Meanwhile, at UNIT HQ Liz Shaw is drawing some conclusions. First, the meteorite is not strictly a meteorite as it has been manufactured in some form of plastic. Fierce scorch marks comply with the theory of it travelling through the Earth's atmosphere but not conclusively. As for it being part of an alien invasion and the story of a Doctor flying through space in a London Police Telephone Box, sheer rubbish!

At the plastics factory Hibbert seems disturbed over Ransome's dismissal but a few reassuring words from Channing seems to set him right. It is almost as though Channing has some kind of mind control over Hibbert. The former says that two energy units are still missing. Their pulsation signals will increase soon to aid recovery.

Within the boundaries of Oxley Woods is Sam Seeley's cottage. In the back yard Sam pulls a heavy metal trunk from inside his shed. Opening it he reveals his "prize" thunderball. As he watches it begins to pulsate more brightly and the warbling noise increases also. In the woods nearby a figure stops as though listening. As it turns, the blue overalled mannequin sets off in the direction of the Seeley cottage.

Interrupting his wonderment Sam's wife Meg returns home. He quickly hides the thunderball and she suspiciously asks what he is about. Satisfying herself that all is in order she goes into the cottage. Sam bundles the meteorite back into the trunk, slams the lid and pushes it back in the shed. As the lid slams shut the blue overalled figure in the woods seems to lose its direction.

At UNIT HQ General Scobie, in charge of the regular troops, pays the Brigadier an informal visit on his way to the Plastics factory. He is to have a plastic replica made of himself for the military display at Madame Tussauds. While in the UNIT laboratory General Scobie cannot help but to see the TARDIS d enquires what they are doing with an obsolete Police Box. Liz is unable to resist telling him that it is really a spaceship!

Back at Ashbridge Cottage Hospital the Doctor awakes to find himself alone. Quickly he steals out of the ward and finds a changing room in which he hopes to find some clothes. As he searches a top notch Doctor enters with Dr Henderson. The visitor has travelled down from London especially to see the Doctor. With nowhere to hide the Doctor dives into the shower and breaks into song. When the coast is clear he emerges and gets dressed in some stolen clothes finishing off the outfit with the Specialist's hat and cloak.

In Oxley Woods the searching troops have had some luck and found a complete meteorite. It is put on board a Land Rover and driven to UNIT HQ. As the Land Rover passes along the road the mannequin in the blue overalls senses the signals from the globe and runs to intercept the vehicle.

The Doctor successfully escapes the hospital and surveying the car park chooses the old red Packard in which to make his escape. Ironically the car belongs to the London big-wig.

On hearing of the Doctor's escape the Brigadier is not unduly concerned. After all, he has the TARDIS and the key! Even if he cannot get the wretched doors open!

The blue overalled figure finally manages to catch up with the Land Rover and promptly steps out directly in font of it. The driver swerves to miss the figure and collides with a tree, killing him instantly. The mannequin helps himself to the meteorite.

This news is somehow relayed to Channing who is waiting at the plastics factory with Hibbert for the arrival of General Scobie. All around them is a mass of new, highly sophisticated machinery.

The Doctor finally turns up at UNIT HQ by the tuse of a TARDIS homing device within his wrist watch. The Brigadier has a barrage of questions which are all dispelled by the Doctor's comment; "I'm sorry, Brigadier, but I've lost my memory!" Side tracking, the Brigadier gets the Doctor's opinion on the meteorites. He immediately states the obvious that they are not meteorites at all. From the shape of the pieces they must have been hollow and therefore contained something. As no whole ones have been found it seems logical that they have been collected by someone else.

The fingers of the outstretched mannequin's hand drop away to reveal a nozzle from which spits an energy bolt. It misses Ransome by inches, scorching pan of the equipment. A game of "hide and seek" around the machine results in Ransome escaping down the fire escape, hotly pursued by the plastic figure. Almost at the perimeter, Channing, Hibbert and General Scobie walk into view. The mannequin stops under a glaring look from Channing. The blue overalled figure, an Auton, turns and disappears into one of the buildings allowing Ransome to escape. Channing and Hibbert see General Scobie out, informing him of a preview of his replica before it goes on display. As soon as the General has left Channing instructs the Auton in home in on Ransome's brain waves, find and destroy him.

A very frightened and tired Ransome stumbles into the UNIT post set up in Oxley Woods. He blurts out his unbelievable story to some amazed soldiers.

In the Seeley household Meg confides in her husband that she is worried over recent events. Sam tells her not to be stupid and, to stop worrying. He too has been taking a keen interest in all the local activities and it occurs to him that his prize "thunderball" may well be worth a lot now.

Unsure of what to make of Ransome the Captain has him sent to the Brigadier. Walking dummies is more his cup of tea.

Back in the UNIT laboratory Liz and the Doctor have been unable to identify the meteorite fragments with the Doctor explaining about the primitive equipment at his disposal. He explains to an incredulous Liz that he has an entire laboratory inside the Police Box, in which, is what they require right now. A Lateral Molecular Rectifier. If Liz could only get the key from the Brigadier. Believing the Doctor completely mad, she agrees to procure the TARDIS key if only to prove the Doctor's insanity!

Arriving at the temporary UNIT post Sam Seeley sheepishly asks Captain Munro is there is a reward for finding one of the "thunderballs"?

In Ransome's old workshop Channing explains to Hibbert that their wuarry is temporarily out of range but as soon as he returns he will be dealt with. More importantly everything is ready for the final 'Phase'. Only one energy unit remains to be collected, the swarm leader. Unbeknown to Channing this is the unit that Sam Seeley is concealing.

At UNIT HQ Ransome relates his story to the Brigadier when Liz enters to ask for the key. The Brigadier tells her to come back later as he is busy. As she turns to leave she spots the key on the desk and expertly palms it on the way out. Within the minute the Brigadier notices the key has gone and rushes off after Liz.

In the laboratory Liz gives the Doctor the key. Thanking her very much he goes inside just as the Brigadier arrives. Both he and Liz watch as the Police Box emits strange wheezing noises followed by an explosion and billows of smoke as the Doctor re-opens the doors. As he steps out he mutters something about being marooned on Earth as the Time Lords have changed the dematerialisation codes. Not understanding a word of what the Doctor is babbling about they all go to see Ransome. Hearing the story once again the Doctor, Liz and the Brigadier plan to visit the factory themselves.

Meanwhile Sam Seeley has confessed to having a "thunderball" and Captain Munro arranges to have it collected.

Meg Seeley has become concerned over the prolonged absence of her husband and goes down to the garden shed to see if he is there. Finding the trunk blocking the doorway she tries to move it but it is too heavy. Opening it she finds the energy unit still glowing and chirruping. In the woods the patiently waiting Auton picks up the signals and starts moving towards the cottage.

Before long Meg hears someone crashing about in the cottage. She thinks Sam has returned but is terrified at what she sees. The Auton has arrived. Running back to the shed she grabs Sam's shotgun and fires point bland at the Auton, hitting it squarely in the chest. It keeps coming!

In the factory Channing is guiding the Auton and orders it to search for the sphere. On route to the factory the Brigadier, Liz and the Doctor stop at the Seeley cottage to investigate the commotion. Encountering the Auton the Brigadier and Captain Munro fire their guns at it but to no effect. Outnumbered, Channing recalls the Auton leaving the energy unit behind which the Doctor soon locates.

The retreating Auton suddenly stops. Picking up Ransome's brain waves, obeying its pre-programmed orders advances on the UNIT tent. Ripping open the back of the tent the Auton enters and fires two energy bolts at Ransome, totally obliterating him. Mission accomplished the deadly mannequin leaves silently.

At last the Brigadier and party arrive at the Plastic factory and see Hibbert. He laughs at Ransome's absurd story saying it is probably an invention to cover his dismissal. The Doctor asks what it is they make now. Plastic mannequins for shop window displays!

Back in UNIT HQ the Brigadier decides to ask for extra troops to surround the factory, pending investigations. He phones General Scobie while the Doctor and Liz are getting excited over detecting an intelligence within the energy unit.

At UNIT headquarters the Doctor and Liz are making some headway. They have asserted that the contents of the sphere are only part of a gestalt without any physical form, explaining the need for the plastics factory. The entity exists on a specific frequency. The Doctor and Liz are working on a machine to establish this frequency with the aim of communicating with their adversary.

The Brigadier receives a call from General Scobie placing the Plastics factory off limits. He promptly hangs up before the Brigadier can argue. He informs the Doctor and decides to go over the General's head and contact Geneva. The Brigadier also tells the Doctor of the General's visit to the factory for the construction of a plastic replica for Madame Tussauds. This greatly interests the Doctor and both he and Liz plan a visit.

At the waxworks the Doctor checks with the attendant that the new models are in fact made of plastic and not wax. Looking round they find General Scobie's replica on show. The Doctor studies the model. It is perfect, even down to the detail of a fully wound wrist watch showing the correct time - in fact this is the real Scobie! The Doctor learns that Channing has replicas of many major political figures which are to infiltrate and take over on his command.

Meanwhile the fake Scobie removes the final energy unit from UNIT HQ and delivers it to Channing.

Back at UNIT HQ, the Doctor and Liz build a machine that they hope will disable the Autons - it is a variation on an ECT machine. All over the country, shop dummies come alive and burst from department store windows, killing anyone in their way.

Hibbert, struggling to break free of Channing's influence, tries to destroy the new machinery installed in the plastics factory. Channing has him killed.

Meanwhile the Doctor, Liz and UNIT break into the factory where they meet with the replica Scobie. The Doctor tests his machine on the fake general. It works and so the Doctor and Liz head off to find the control centre. Taking charge of the regular troops as well as his own, the Brigadier is ready to take on the advancing party of Autons. A fierce battle ensues with the contemporary weaponry ineffective against solid plastic.

Liz and the Doctor, on the other hand, are having more success. Ably dealing with the occasional Auton they soon reach Ransome's old workshop, the nerve centre of the invasion. Seeing the Doctor, Channing turns the machine tank to full power. The seething mass within seems almost to burst out. The Doctor approaches the alien and tells Liz to switch on. Nothing happens. A fault has developed. His attention distracted a writhing tentacle appears from the top of the tank, envelops the Doctor's neck and starts to strangle him.

Frantically Liz rectifies the fault, a loose connection, and the machine hums into life. Slowly, at first, the Nestene creature releases the Doctor. He and Liz watch its death throws before switching off. On the far side of the room is the crippled remains of Channing. A gestalt, being a communal creature, as the main entity is dead so all the other related creatures 'die'. Outside all the fighting Autons suddenly collapse as do all the replica Government Ministers, etc. The invasion is over.