At a circus, a horse box materialises out of thin air and the Master (Roger Delgado) emerges. He hypnotises the circus owner, Luigi Rossini, whose real name is Lew Russell (John Baskcomb), and uses him to steal a Nestene energy unit from where it is on display in a museum, on loan from UNIT HQ. 

At UNIT HQ the Doctor meets his new assistant, Josephine Grant (Katy Manning), as Liz Shaw has returned to Cambridge. Despite his initial apprehensions, the Doctor cannot bring himself to tell the enthusiastic Jo that he doesn't want her help. 

The Master enters the Ministry of Technology's Beacon Hill research establishment and, killing Goodge (Andrew Staines), one of the scientists, and hypnotising another, Philips (Christopher Burgess), he uses the radio dishes to send a signal into space. The energy unit starts flashing as the Nestene Consciousness once more inhabits it. 

Captain Mike Yates (Richard Franklin) reports to the Brigadier (Nicholas Courtney) about the missing scientists at Beacon Hill. There, the Doctor is warned of the presence of the Master on Earth by another Time Lord (David Garth). The Time Lord also alerts the Doctor to the presence of a bomb in the control room. The Doctor manages to prevent the bomb from exploding. 

Back at UNIT HQ, the Brigadier sets up a search of all plastic factories in the hope of finding the Master. Meanwhile, the Master, posing as Colonel Masters, has hypnotised Rex Farrel (Michael Wisher), owner of a plastics factory, and is arranging for the production of Nestene-controlled plastic Auton products. He kills James McDermott (Harry Towb), the factory's chief engineer, with a black plastic chair that suffocates him, but McDermott has already alerted John Farrel, Rex's father and the retired manager of the factory. 

Jo, investigating on her own, is caught and hypnotised by the Master, and made to open a box back at UNIT HQ found in Philips' car. The box contains a bomb and the Doctor realises the danger in time and throws it out of the window where it explodes harmlessly. The Doctor manages to break Jo's trance but she cannot remember where the Master was based. 

John Farrel (Stephen Jack) arrives to see Rex. The Master gives him an ugly plastic troll (Tommy Reynolds) as a present. Activated by heat, it comes alive in his house and strangles Farrel to death. 

Yates reports that a circus had recently been at the place where Philips' car was found and it has now moved to Tarminster. The Doctor goes there to see if anyone has seen Philips and Jo stows away in Bessie. The Doctor is seen investigating the Master's horse box - his TARDIS - and Rossini takes him prisoner with the help of Tony the Strongman (Roy Stewart). Jo sees Philips go into the horse box and alerts UNIT before rescuing the Doctor. Philips confronts them clutching a grenade and the Doctor tries to break through the Master's hypnotic conditioning. Philips rushes outside where he is killed as the grenade explodes. The Doctor takes a key from Philips' hand and steals the dematerialisation circuit from the Master's TARDIS. 

A mob of circus folk, led by Rossini, attack the Doctor and Jo. They are saved by two policemen (Terry Walsh, Dinny Powell) who bundle them into their car and speed off, just as the Brigadier and Yates arrive. The policemen drive the Doctor and Jo to a quarry where they are revealed as Auton dummies. Following the police car, the Brigadier rescues the Doctor and Jo from the Autons. Back at UNIT HQ, the Doctor discovers that the Master's dematerialisation circuit does not work in his TARDIS and realises that the Master is now trapped on Earth as well. 

Brownrose (Dermot Tuohy) arrives from the Ministry with a report of sudden and unexplained deaths occurring all over the home counties. The first two were McDermott and Farrel from the same plastic factory. While the Doctor is out seeing Farrel's widow (Barbara Leake), the Master, disguised as a telephone engineer (Norman Stanley), fits an extra-long cord to the telephone in the Doctor's lab. Back at the lab, the Doctor, Jo and Yates discover that the doll given to them by Mrs Farrel comes alive when heated, and the Doctor and the Brigadier decide to visit the plastics factory. It is deserted barring a plastic daffodil on the floor and an Auton in the safe. 

Back at UNIT HQ, the Doctor is nearly strangled by the new telephone cord when the Master activates it. The Brigadier arrives and saves the Doctor. UNIT have located the bus being used by the factory to distribute the daffodils as part of a plastic promotion - it is parked in a quarry. He orders an air-strike to destroy it. 

The Doctor discovers that the daffodils are activated by short wave radio signals and that they spray a fast-hardening plastic over the nose and mouth of anyone close enough, causing death by asphyxiation. The plastic seal dissolves with the victim's final breath. The Master arrives and takes the Doctor and Jo hostage to prevent the Brigadier from destroying the bus. The Master intends to activate the daffodils by radio impulses from the Beacon Hill centre. 

The Doctor and Jo escape from the bus and the Doctor chases after the Master while UNIT battles with the Autons. The Doctor manages to convince the Master that the Nestenes will never recognise him as their leader. The Master, realising that the Doctor is right, helps him repel the Consciousness back into space. 

As the Autons all collapse, the Master runs for the bus. He emerges minutes later and Yates guns him down as he pulls a gun from his jacket. They discover that the body is that of Rex Farrel - he was wearing a rubber mask of the Master's face. The Master escapes in the bus.

_{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}