At a circus, a lorry materialises out of thin air with the distinctive sound of a TARDIS. The occupant is a thin, bearded man dressed in a black suit who introduces himself as the Master to the circus owner, Rossini (though his real name is Russell), and hypnotises him. Subsequently, the Master and Russell break into the National Space Museum and steal a translucent plastic polyhedron, one of the energy units used by the Nestene Consciousness in their attempted invasion of Earth. The Master then takes the energy unit to a radio telescope facility, killing the technician on duty with a weapon that shrinks the victim, leaving a twisted, doll-sized body. The Master then hooks up the energy unit to the radio telescope and sends a signal into space.
At UNIT headquarters, the Doctor meets his new assistant, a young, enthusiastic but slightly scatter-brained trainee named Josephine Grant. Dismayed at first that he is not getting a scientist to replace Liz Shaw, who has returned to Cambridge, he reluctantly accepts her when he hasn't the heart to tell her otherwise. Reports of the theft of the Nestene unit and sabotage at the radio telescope facility lead the Doctor, Brigadier Lethbridge-Stewart and Jo to investigate. At the facility, the Doctor encounters a fellow Time Lord who warns him that his old enemy, the Master, is here and will try to kill him. The Master, in the meantime, has hypnotised Farrel, the young manager of a plastics factory. Under the name of Colonel Masters, he takes over the factory's production to build Autons. McDermott, Farrel's assistant, gets suspicious of Masters and calls up Farrel's father, the owner of the factory.
Jo, investigating Farrel's factory, is discovered by the Master who wipes her memory of meeting him and sends her back to UNIT. When the chained-up box that used to contain the unit is brought to UNIT headquarters, Jo offers to open it. The Doctor realizes that Jo has been hypnotised and that the box contains a bomb.
The Doctor manages to throw the open and smoking box out the window, where it explodes in the river. Jo falls into a catatonic state from which the Doctor revives her, but she is unable to remember where she met the Master. At the factory, McDermott confronts the Master, and is killed by a plastic chair that swallows him up and suffocates him. The elder Farrel arrives, threatening to retake the factory, but his will is strong enough to resist the Master's hypnotism. The Master sends the elder Farrel home with a plastic doll of a troll, which comes to life when placed next to the radiator and strangles him.
Searching for Phillips, the other scientist missing from the facility, the Doctor visits Rossini's circus. He is captured by Rossini just as he is about to open the Master's TARDIS and tied up. He is freed by Jo, who had followed the Doctor there against orders. Phillips, also under the Master's influence, tries to kill Jo and the Doctor with a grenade. The Doctor urges Phillips to resist, and he is killed while trying to throw the explosive away. The Doctor enters the horsebox and removes something from it, only to be attacked by Rossini and his men. Seemingly rescued by a police car, the Doctor gets suspicious and unmasks them as Autons.
Escaping from the vehicle, the Doctor and Jo hide as the Brigadier and Captain Mike Yates arrive. A firefight breaks out between them and the Autons from which they manage to escape in the Brigadier's car. Back at UNIT, the Doctor fits the dematerialisation circuit he "borrowed" from the Master's TARDIS and tries to take off, but only manages to produce a lot of smoke. His frustration turns into amusement, however, when he realizes that without the circuit, the Master is now trapped on Earth as well.
Meanwhile, Autons appearing like men dressed with big plastic heads hand out plastic daffodils to the public. Soon deaths from asphyxiation, shock and heart failure are being reported across the country. The only connection is between the first two victims - McDermott and the elder Farrel. Interviewing Mrs Farrel, Jo and the Doctor discover the elder Farrel's concerns about "Colonel Masters". The Master, meanwhile, has infiltrated UNIT headquarters disguised as a telephone technician and installs a long, plastic telephone cable in the Doctor's laboratory. The Doctor brings the troll doll back to UNIT to examine it, but it is simply solid plastic. While the Brigadier and the Doctor are at the factory, the doll comes to life due to its proximity to a Bunsen burner and attacks Jo, until Yates shoots it. At the now empty factory, the Brigadier and Doctor discover that Farrel has chartered a bus. They also find a plastic daffodil and an Auton, proving the connection between the factory and the Master.
Back at UNIT, Yates tells Doctor about the doll, but using heat on the daffodil fails to activate any sinister function. The telephone rings, and it is the Master, who bids the Doctor goodbye. The Master sends an electronic signal across, animating the telephone cable which then tries to strangle the Doctor.
Luckily the Brigadier hears the Doctor's cries for help and disconnects the cable. The Brigadier calls out an airstrike on the Auton bus. As the Doctor tries to decode the Nestene instructions imprinted in the plastic flower, a radio signal from a walkie-talkie accidentally activates it. The daffodil sprays a plastic film over Jo's face, nearly suffocating her until the Doctor removes the film with a spray. The plastic quickly dissolves soon after, explaining why it was not found at the sites of the deaths.
The Master arrives at UNIT to retrieve his dematerialisation circuit, threatening to kill Jo if he does not hand it over. Jo, trying to convince the Doctor not to, foolishly blurts out that the airstrike has been confirmed. With this revelation, the Master decides to bring Jo and the Doctor to the Auton bus parked in a quarry. The Brigadier has no choice but to abort the airstrike, and the bus drives off to the radio telescope. Farrel, regaining his wits at last, tries to crash the bus in a field, and the Doctor and Jo escape. UNIT troops engage the Autons while the Doctor and Jo pursue the Master into the facility's control room where the Master is opening the radio frequency for the Nestene invasion force to come through. However, the Doctor convinces the Master that the Nestenes will not distinguish between ally or foe once they arrive. Together, they close the channel for the invasion, driving the Nestenes back to wherever they were coming from and causing the Autons to collapse. While the Doctor and the Brigadier catch their breath, the Master vanishes.
At the bus, the Master emerges, apparently surrendering, but when he pulls out a pistol, Yates shoots him. The Doctor peels back the disguise on the corpse to reveal that it is Farrel made up look like the Master, as the real Master drives off in the bus. However, with the dematerialisation circuit in the Doctor's hands, the Master is still trapped on Earth. The Doctor remarks to Jo that he looks forward to their next encounter.
Doctor Jon Pertwee (Third Doctor)
Companion Katy Manning (Jo Grant)
Nicholas Courtney — Brigadier Lethbridge-Stewart
Richard Franklin — Captain Mike Yates
John Levene — Sergeant Benton
Roger Delgado — The Master
David Garth — Time Lord
Michael Wisher — Rex Farrel
Stephen Jack — Farrel Snr.
Barbara Leake — Mrs Farrel
Harry Towb — McDermott
Christopher Burgess — Professor Phillips
Frank Mills — Radio Telescope Director
Andrew Staines — Goodge
Dermot Tuohy — Brownrose
John Baskcomb — Rossini
Roy Stewart — Strong Man
Dave Carter — Museum Attendant
Bill McGuirk — Policeman
Pat Gorman — Auton Leader
Terry Walsh — Auton Policeman
Haydn Jones — Auton Voice
Writer Robert Holmes
Director Barry Letts (uncredited)
Script editor Terrance Dicks
Producer Barry Letts
Executive producer(s) None
Production code EEE
Series Season 8
Length 4 episodes, 25 minutes each
Originally broadcast January 2–January 23, 1971