The TARDIS materializes in what seems to be the English countryside, but the Doctor detects a strange energy reading. He and Sarah meet a group of men in white suits and opaque helmets who shoot at them out of their index fingers. Trying to escape, they see a UNIT soldier, twitching madly, run over a cliff to his death. Searching his body, the Doctor finds a wallet of freshly minted coins, all dated the same year. There is also a casket-shaped pod nearby. Still pursued, they find a deserted village, which Sarah believes is Devesham, located near a space defence station. The White Suits enter the village, accompanied by the dead soldier. Villagers in a trance state arrive and take positions. When the clock strikes, they come to life.
The Doctor wants to get to the Space Defense Station and contact UNIT. The soldier questions Sarah. Morgan (Peter Welch) suggests Sarah is part of “the test” and they let her go. She can see there is nothing but plastic and electronics behind their visors. Examining a pod near the TARDIS, she sees the TARDIS vanish, and a man in the pod attacks her, but she escapes.
At the Defence Station, Senior Defence Astronaut Guy Grayford (Milton Johns) is told by the voice of Styggron (Martin Friend) to check for an intruder. The Doctor introduces himself as UNIT’s scientific advisor but is locked up. Sarah arrives and frees him, but an alien named Kraal (Stuart Fell) is watching them. Sarah tells the Doctor that Crayford was supposed to be lost in deep space. The Doctor escapes, but Sarah is captured. In an alien room, she is strapped to a table and scanned by a simulacrum of Harry. Styggron orders the final test of the Earth invasion.
In the pub, the Doctor is phoned by Sarah, though he knows the phones don’t work. He is surprised to find that the TARDIS is gone, but realizes it is on its way to the real Earth. Further alerted by the fact that Sarah is wearing a scarf that he took from her hours ago, he knocks her down and her face falls off, revealing the mechanism inside. She fires at him, but he escapes.
Styggron orders the village and the Doctor destroyed by a bomb. He is captured, tied up, and placed beside the bomb in the town square. The real Sarah escapes and frees him; they run to the base and hide inside as the bomb goes off and the village dissolves, but they are captured by androids. The Doctor explains that the radiation he detected in the beginning shows they are on Oseidon, the Kraal planet, which will soon be uninhabitable. Crayford is helping the Kraals because they saved him when Earth left him for dead in space, but now he knows they intend to wipe out the human species. The Doctor and Sarah escape to Earth in Crawford’s rocket and land in pods, but an android version of the Doctor is also on the way. The real Benton and Harry have found the TARDIS, uninhabited, in the real Devesham.
Sarah and the Doctor land on Earth in their pods, but so do their android replicas. The Doctor tries to explain the Kraal invasion to UNIT, but Harry and Colonel Faraday (Patrick Newell) have already been replaced and the android Doctor pulls a gun on him. He escapes, but bluffs his way back in again, posing as his android double. Crayford’s rocket had been hijacked by the Kraal and he was not reconstructed but brainwashed. In the final struggle, Crayford is killed, Styggron is killed, and the Doctor is killed. But then the real Doctor appears.
The serial had a mixed reception, reviewers noting that it too much resembled Invasion of the Body Snatchers, The Stepford Wives, and episodes of the Bionic Woman, Star Trek, UFO, and Space 1999. There is nothing new under any sun. It was called silly, far-fetched, and stupid. I have seen some stupid Doctor Who, and I don’t think this qualifies. Like many, I found the Twilight-Zone-like village intriguing and considered much of it to be a fast-paced adventure mystery. There were some major logical flaws in the plot—mostly not noticed by fans until the video recordings came out--and some characters seemed awfully naïve or illogical. The stone-like faces of the Kraal were unfortunate for a species that spoke so much technological jargon. This was the last appearance of Ian Marter in Doctor Who. A diabetic, he died in 1986, at the age of 42.