It is an ordinary day at a UNIT Tracking Station when a radar technician is suddenly alerted to several strange meteorites appearing, seemingly from nowhere, on his radar screen. Disbelieving at this formation he watches in amazement, as they seem to land on earth.

Elsewhere in Oxley Woods, a poacher by the name of Sam Seeley narrowly avoids being hit by several of the meteorites careering towards their landing places embedded in the earth. He soon regains his courage and investigates these strange proceedings. He takes one of the ‘meteorites’, an oddly shaped spherical object emitting a pulsating glow and high-pitched sound. However when touching it with his hands he realises it is still burning hot and instead he buries it, intending to return for it at a later time. Nearby the quiet of the woods are disturbed once more, this time by the sound of strange alien engines and, sure enough, the TARDIS soon materialises. The doors of the time machine open and out of them falls a somewhat shabby and unfamiliar Doctor.

Later that day, after receiving a communication from Brigadier Lethbridge-Stewart, scientist Liz Shaw attends a secret rendezvous with the UNIT commander regarding the post of Scientific Advisor. She is however sceptical of the alien experiences of UNIT and at first refuses to join the organization. But when Lethbridge-Stewart explains about the meteor showers, one of which we observed earlier, there have in fact been many more in exactly the same place, she becomes interested by the proposal.

Shaw and the Brigadier are interrupted by an urgent call from UNIT’s Captain Munro. He and his troops have been searching Oxley Woods for the meteorites and have found a Police Box with an unconscious man lying next to it. These words spur Lethbridge-Stewart into action. Could this man be the Doctor he has been telling Liz about? He orders a guard to be placed on the TARDIS and himself and Liz travel to the hospital where the man has been taken.

The Doctor’s anatomy is causing much confusion at the hospital. Dr Henderson takes X-Rays only to discover the Doctor apparently has two hearts. Meanwhile Dr Lomax is baffled by the blood sample Henderson took from the Doctor, discovering it is not human blood. Mullins, the hospital porter, overhears a conversation between the two doctors and sells the story to the Daily Chronicle.

Meanwhile, Sam Seeley has returned to Oxley Woods and has collected his meteorite. Putting it in a sack he heads off home, trying to avoid the nearby soldiers.

The Brigadier and Liz arrive at the hospital to find a press conference taking place on the subject of the hospital’s ‘alien patient’. The Brigadier is furious and pushes his way through the crowd. However when he gets there he finds that the man is not in fact the Doctor, or at least not the same one he has encountered before. However as he turns to leave the room the Doctor sits up and greets the soldier as an old friend. The Time Lord borrows Liz’s mirror and, on seeing his new face, slips back into unconsciousness. The Brigadier is baffled by this strange behaviour and orders he be taken to UNIT HQ as soon as possible.

The Brigadier speeds away from the hospital in his car. The reporters see this and rush to use the phone, but in the hospital phone box one man is standing, staring straight ahead as if in a daydream. When the door is opened he turns and walks away without a word.

Back in the woods Seeley is caught by two UNIT soldiers and when questioned acts the fool and is let go with a warning.

The Doctor meanwhile has awakened and demands his shoes from Dr Henderson. The nurse goes off to fetch them and the Doctor clutches them to his chest before dozing off once more. Dr Henderson and the nurse worry about this seemingly mental behaviour and they leave to fetch some presumably rather expensive equipment. Once they’ve gone the Doctor awakens once more and takes a key concealed in his right shoe. He clutches this tightly in his fist and slips back into his slumber, seemingly relieved and happy.

Soon afterwards Dr Henderson sits at the Doctor’s bedside taking notes. Two men come up behind him and hit him over the head, knocking him out. They put the Doctor in a wheelchair and put Elastoplast over his mouth. They take him out towards an ambulance waiting for them outside. As he boards the ambulance two strangers come in and turn the wheelchair around before running away down the drive. The two original kidnappers board their ambulance and are driven off by the strange man who was earlier standing in the phone box.

UNIT’s Captain Munro has by this time found the unconscious Dr Henderson but he’s too late to get to the ambulance before it speeds away. The UNIT soldiers shoot at the tyres but fail to bring the vehicle to a halt.

Luckily the Doctor isn’t seriously wounded. Dr Henderson takes him back into the hospital but an ECG scan result shows that there is little brain activity in the patient, which he theorises, is self-induced. The Brigadier and the others still don’t realise that this is the Doctor although after attempting to get to the TARDIS he begins to think that it is possible, but how? Dr Henderson also retrieves a key from the Doctors hand. Could this be the TARDIS key?

The TARDIS is moved to UNIT HQ in order to be guarded. The troops in the woods have also found fragments from one of the ‘meteorites’, which Miss Shaw is asked to examine. Captain Munro has developed a photo of the man that led the kidnappers who tried to take the Doctor, which the Brigadier uses to instigate a search.

Meanwhile at a factory, Auto plastics Ltd, a designer for the company named Ransome arrives back from a business trip to the USA, only to find the factory oddly empty. After going to his workshop (which is now labelled as ‘Out Of Bounds’), he immediately goes to Hibbert, a friend and the managing director. Hibbert coldly tells him that he has been dismissed and that his workshop now has other uses. Ransome protests but eventually leaves, unaware Channing, the new partner of Hibbert at the factory and the leader of the kidnappers at the hospital, is watching him.

At UNIT HQ Liz Shaw has deduced that the object from the woods isn’t a meteorite; it is made of plastic. The theory of it having travelled through the earth’s atmosphere is significantly helped by the fierce scorch marks that the pieces carry but Liz still doesn’t believe they are part of an alien invasion. She also snubs the Brigadier’s claims that the Police Box found in the woods can fly through time and space with the alien Doctor inside.

Back at Auto Plastics Hibbert seems shaken, but Channing reassures him, appearing to have some sort of mind control over him. Hibbert almost reluctantly tells him that there are two ‘energy units’ missing and Channing informs him that their energy signals will soon increase, which means they will be able to find them more easily.

Elsewhere, Sam Seeley is now at home in his cottage. In the shed round the back of the building he is handling a large and heavy metal trunk. In it is the ‘meteorite’ that he found earlier in the woods. As he watches it, it begins to pulse brightly and make a high-pitched warbling noise. In the woods nearby, a strange figure resembling a mannequin in blue overalls stops and listens; suddenly setting off in the direction of the Seeley cottage.

Sam’s wife Meg returns to the house, breaking Sam’s wondrous state. He hides his meteorite as she eyes him suspiciously. He puts the globe back in the trunk and slams the lid down. With this the mannequin seems to lose its direction slightly, beginning to stumble around.

General Scobie pays the Brigadier a visit at UNIT HQ, informing him that he is to have a replica of himself made at Madame Tussauds in the near future. He notes the large blue box in the room and enquires as to what it is. Liz cannot help herself, and explains to him that it is really a spaceship.

At the hospital the Doctor has finally awoken again and finding himself alone he goes into a changing room nearby. No sooner has he started to search for some clothes. Dr Henderson enters with a top-notch doctor visiting from London, specifically to see the Doctor. With nowhere else to hide the Doctor dives into the shower and starts singing. When the coast is clear he dons some stolen clothes including the visitor’s hat and cloak.

In the woods some UNIT troops have found another ‘meteorite’. As their Land Rover drives through the woods the mannequin runs into the road. The driver swerves to avoid what he thinks is a person and crashes into a tree, dying instantly. The mannequin ignores the lifeless body, and takes the meteorite.

The Doctor has escaped the hospital in the specialists’ vintage car. On hearing this news the Brigadier doesn’t appear to seem all that perturbed. He now has the TARDIS and its key but he cannot open the doors for reasons unknown.

The news of the mannequin’s actions is somehow relayed to Channing. who is standing with Hibbert in a room filled with lots of complex machinery. They are waiting for the arrival of General Scobie, of whom they are creating a waxwork replica for display in Madame Tussauds. The Doctor meanwhile has arrived at UNIT HQ and refuses to answer the Brigadier’s questions; claiming he has lost his memory. He does however give his opinion on the meteorites and, like Liz, deduces that they are not meteorites. He adds to his assumptions that whatever they are there were several more of them, and someone must be collecting them up…

The fingers of the mannequin drop down from its hand on a hinge revealing the muzzle of a gun. From this comes an energy bolt that Ransome narrowly manages to avoid, hitting some of the machinery instead. He manages to escape down a fire escape at the side of the building, pursued by the mannequin.

Channing, Hibbert and General Scobie come into view. At a glance from Channing the mannequin breaks off chase and Ransome gets away. The mannequins are Autons and Channing uses a brain relay system to instruct the Auton to find Ransome and destroy him. By this time General Scobie has left, having been promised a preview of his replica before its display.

Ransome is visibly frightened and tired when he arrives at the UNIT base in the woods. He tells his tale to some disbelieving soldiers.

At the Seeleys home Meg informs her husband Sam of her worry over recent events in the woods concerning the soldiers. Sam tells her rather curtly not to worry but he too has been taking an interest in the local affairs. He has realised that somebody must be looking for his ‘meteorite’. It could be worth a lot of money.

Captain Munro is unsure of Ransome and so sends him to see the Brigadier. Meanwhile in the UNIT lab the Doctor and Liz have been unable to find out much more about the mysterious ‘meteorites’. The Doctor complains about the primitive Earth equipment he is having to use and tells Liz that he has all the necessary equipment inside his TARDIS, tricking her into stealing the keys from the Brigadier’s office.

Sam Seeley arrives at the UNIT outpost in the woods carrying a sack and inquiring how much the soldiers will give for someone who finds a ‘meteorite’.

Channing is calm about the operation he is conducting, with the exception of the fact that one of the required energy units is still missing.

The Brigadier and Ransome begin to talk. Liz enters and asks for the key to the TARDIS but the Brig tells her to go away. She spots the key on his desk and once he is not looking she steals it. However he realises almost straight away and rushes after her, arriving just in time to see the Doctor thank Liz and go into the TARDIS, shutting the door after him. The two humans listen as the TARDIS struggles to take off and observe as smoke fumes arrive as a visual accompaniment.

The Doctor steps out of the machine muttering about ship’s dematerialisation codes being changed by the Time Lords and thus becoming effectively trapped on Earth. Having finished his outburst the three go and talk to Ransome and the Doctor eventually decides that they should visit the factory.

Back in the woods Sam Seeley has confessed to hiding the ‘meteorite’ and he arranges with UNIT for it to be collected.

At the cottage Meg Seeley is concerned about her husbands’ long absence and, thinking he may be in the garden, goes outside. She finds the trunk and opens it. In the woods the waiting Auton stops and starts walking towards the house. Meg hears someone crashing about in the house and thinks Sam has returned, probably drunk. She goes inside and instead sees the Auton. Rushing back outside she uses Sam’s shotgun to shoot it squarely in the chest at almost point blank range. But the Auton keeps on coming.

Channing is guiding the Auton from the factory using his mind. He orders it to search for the sphere in the Seeley household. En Route to the Auto Plastics factory the Brigadier, the Doctor and Liz stop in at the cottage to find out the source of the disturbance. The Brig and Captain Munro fire their guns at the Auton but to no effect. Despite this Channing recalls the Auton due to it being outnumbered. Once it has gone the Doctor finds the sphere in the trunk.

However the Auton still has work to do, and has now picked up Ransome’s brain waves. Finding him in the UNIT tent it disintegrates him with two energy bolts.

The Brigadier and co now arrive at the factory where they retell Ransome’s story but Hibbert laughs it off. He claims that Ransome is just trying to get his own back for being made redundant by the factory. The Doctor asks Hibbert what the factory makes and he explains that they manufacture mannequins for shop windows.

The Brigadier orders UNIT soldiers to surround the factory. He contacts General Scobie while the Doctor and Liz make a breakthrough with their sphere. They have found an intelligence contained inside it.

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.