The TARDIS arrives on Earth shortly after a shower of about fifty meteorites which had been spotted and tracked by a UNIT base as they appeared to be flying in formation. The Doctor collapses outside the TARDIS where he is found by UNIT troops searching for fallen meteorites and taken to the nearby Ashbridge Cottage Hospital. There, Doctor Henderson (Antony Webb) discovers that his patient appears not of this world: his x-rays show that he has two hearts and his blood is not of an identifiable human type as shown by the platelets' stickiness. This information is overheard by Mullins (Talfryn Thomas), the hospital porter, and he telephones it through to the Daily Chronicle. Soon the hospital is besieged by newspaper reporters. 

Meanwhile Brigadier Lethbridge-Stewart (Nicholas Courtney) of UNIT has interviewed and accepted a new scientific adviser to UNIT in the form of Doctor Elisabeth Shaw (Caroline John), formerly in charge of a research programme at Cambridge, who is also an expert in meteorites and who has degrees in medicine, physics and a dozen other subjects. The Brigadier and Liz travel to the hospital to see the stranger as the Brigadier feels it may be the Doctor. Although the man looks totally different, he recognises the Brigadier. As the Brigadier is leaving, the Doctor is kidnapped by two men while a third, Channing (Hugh Burden), supervises. The Doctor manages to escape in a wheelchair, but is shot by the UNIT men guarding the TARDIS as he tries to make his way towards it. Back at the hospital, Henderson discovers that the Doctor's EEG reading is now completely flat. 

Channing has taken control of a plastics factory and has been collecting the meteorites - which he calls energy units - but there are still two missing. Channing is controlling George Hibbert (John Woodnutt), the former boss at the factory, and Hibbert has made redundant a disgruntled John Ransome (Derek Smee). Ransome secretly returns to the factory and narrowly escapes being killed by a walking plastic mannequin with a gun concealed in its hand. 

The Doctor recovers in hospital and escapes, stealing the clothes and car of a visiting doctor in the process. He heads for UNIT HQ where he agrees to help the Brigadier and Liz in exchange for the TARDIS key, which the Brigadier has custody of. The Doctor persuades Liz to get the key for him, but he discovers that he can no longer operate the TARDIS and is trapped on Earth. 

Meanwhile, UNIT troops led by Captain Munro (John Breslin) have found a complete meteorite but it is stolen en route for UNIT HQ by one of the mannequins controlled by Channing. The final meteorite - the swarm leader - has been found by a poacher, Sam Seeley (Neil Wilson), and is being kept in a metal trunk at his home, Brook Cottage. He eventually tells UNIT of his find and when they go to his cottage, they disturb a plastic Auton - one of the mannequins - which is also searching for the globe. 

Back at UNIT HQ, the Doctor realises that there is some form of intelligence in the globe, and, with information from a distraught Ransome about moving mannequins, starts to piece together the puzzle. Ransome is killed and vaporised by an Auton and Major-General Scobie (Hamilton Dyce), in charge of the regular Army, is replaced by a plastic replica. The Doctor decides to visit Madame Tussauds and discovers that the real Scobie is there. He also 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. 

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. UNIT troops fight the Autons as the Doctor tests his machine on the replica Scobie. It works and so the Doctor and Liz head off to find the control centre. There they confront Channing, and the Doctor is attacked by a huge octopoid creature - the collective being of the Nestenes who have come to conquer the Earth - which has been created by the machine. The Doctor's weapon is not working, but Liz realises the problem and it is fixed in time to destroy the intelligence and Channing along with it - he was an Auton himself. 

Back at UNIT HQ, the Doctor agrees to stay and help the Brigadier and UNIT in exchange for clothes, a new car, help from Liz Shaw and the facilities to try and repair the TARDIS. The Brigadier agrees and asks the Doctor what his name is. 'Smith,' replies the Time Lord. 'Doctor John Smith.'

Synopsis from Doctor Who: The Third Doctor Handbook by David J. Howe, Mark Stammers and Stephen James Walker, reprinted with permission; further reproduction is not permitted. Available from Telos