The Doctor and Sarah Arrive
The TARDIS materialises in a wood, on what The Doctor and his travelling companion Sarah Jane Smith assume to be Earth. The Doctor uses a device to check and detects an odd reading of energy or radiation nearby. They set off through the wood to explore so as to identify exactly were they are. They have not got very far when The Doctor suspects they are being watched. His suspicions are confirmed correct when a group of white-suited men, wearing opaque helmets, suddenly appear. The Doctor realises that they are androids when they start shooting at them both with guns in their index fingers.
After escaping from the androids The Doctor and Sarah arrive at the top of a cliff overlooking a disused quarry. After rescuing Sarah, who nearly falls down the cliff, they are shocked to witness a man dressed as a soldier, seemingly in a trance, stumble straight towards the cliff. Despite their shouted warnings the soldier walks straight over the cliff edge and plummets to his death at the bottom. On reaching the dead man The Doctor and Sarah are puzzled to discover that he is a member of UNIT and in his pockets he has nothing but newly minted coins – all with the same date. Nearby is a casket-shaped pod, which The Doctor finds familiar. Before he can examine it further, The Doctor and Sarah are shot at by the androids who have arrived at the top of the cliff.
After leaving the disused quarry, and running through the countryside to shaking off their pursuers, they arrive in what Sarah recognises is the English village of Devesham which lies about a mile from a Space Defence Station – the possible cause of the odd reading The Doctor detected when they first arrived - Curiously the village seems deserted. Even in the local pub there is no sign of any villagers. Sarah then spots the androids coming down the street accompanied by the soldier they earlier witnessed killing himself at the quarry. A lorry then arrives, carrying what seem to be villagers, all who seem to be in a trance-like state. They are helped off the vehicle by the androids, and distribute themselves around the village. While The Doctor and Sarah hide in a store room they watch the villagers silently take their seats in the pub, waiting motionless. Then, when a clock chimes, they all suddenly come to life and start to behave normally.
While The Doctor goes to the Space Defence Station, to contact UNIT, Sarah is discovered in the store room by the ‘dead’ soldier who questions her. Sarah manages to escape from the pub and while hiding behind the lorry she observes one of the androids who turns around to face her. Sarah though is shocked to discover that behind the opaque visor is nothing but a slab of plastic and electronics.
Sarah returns to the TARDIS in the woods, as instructed by The Doctor, but the time machine dematerialises of its own accord after Sarah inserts the key in the lock. Nearby is another pod. When Sarah goes to examine it she is startled when it opens and she discovers a man, apparently asleep, lying inside. However, when she gets closer, he suddenly comes to life and grabs her around the throat. She manages to break free and runs into the woods - heading for the Space Defence Station and The Doctor.
At the Defence Station, The Doctor meets Senior Defence Astronaut Guy Crayford who realises that The Doctor is an intruder. After being held at gunpoint, managing to escape and then being recaptured The Doctor is finally rescued by Sarah when she arrives at the Space Defence Station. As they explore further The Doctor and Sarah are pleased when they discover their friend Sergeant Benton is there. But their delight is short lived when they discover that he too is an android. They even discover that there is an android double of Harry Sullivan.
The Doctor realises that they have not arrived on Earth. They have in fact arrived on the planet Oseidon, home of the Kraals, who have created an exact replica of Devesham and its nearby Space Defence Station in order to carry out a rehearsal so as to test their android villagers prior to a planned full invasion of Earth.
The Kraals are being helped by Guy Crayford who is resentful at his fellow humans who he believes abandoned him in space. The Kraals have convinced Guy Crayford that they rescued him and re-built him after a crash, and he discovers too late that this is a lie to gain his trust. Styggron, the Kraal leader, intends to infect the humans on Earth with a virulent virus and to replace prominent figures with his androids causing as much disruption as possible and thus weakening resistance to the planned invasion.
After being captured, questioned, tied up, left to die, and on learning of the TARDIS’ sudden departure without them, The Doctor, along with Sarah, is forced to travel in Guy Crayford's rocket, the XK5 space freighter, when the dummy village is destroyed by the Kraals. However, inside the rocket they discover pods containing androids and realise that the rocket is being used to carry the spearhead of the Kraal invasion.
Arriving on Earth, The Doctor and Sarah try to persuade UNIT troops at the Space Defence Station that there is an invasion imminent, but they discover that The Brigadier is away in Geneva, Guy Crayford is being hailed as a hero and that there are also android duplicates of themselves. With the humans being replaced by android doubles The Doctor uses the Station's transmitters to jam the frequency control the android duplicates.
The Doctor then reprograms his android double and uses it to prevent Styggron from releasing the virus. Styggron however, accidentally infects himself during the fight with the android Doctor and is killed - but not before Styggron shoots the android Doctor. Sarah though is unaware at first that the android Doctor is not the real Doctor.
With the Kraals’ invasion plans foiled The Doctor and Sarah slip quietly back to the TARDIS leaving UNIT to clear up and to deal with the explanations.