The TARDIS lands on a seemingly deserted planet. The Doctor and co. come across a door, which is part of a machine. Abu, a Gond who was selected earlier, emerges and is then obliterated in some sort of gas. They enter the Gond city, just when Thara tries to prevent Vana from being given to the Krotons. The Doctor tries to stop this but too late, she enters a door to be with the Krotons. The Krotons live inside the machine and the Gonds regard it as a great honour to be selected to join them. The Doctor is disbelieved when he says Abu was destroyed; the Gonds think the wasteland is polluted by deadly gases. Thara goes with the Doctor and co. when they rush off to save Vana. Vana is saved but is in a mindless state. The leader begins to believe the story now. Thara questions the Custodian of the Learning Hall in order to lure the Krotons out. The Doctor learns that the Krotons teach the Gonds via leaning machines, that two students at a time are sent to the Krotons, and in the past the Gonds and Krotons, who "fell from the sky" had been at war. Thara and others smash machines but stop suddenly when a Kroton voice is heard, ordering them to leave. A machine in the Kroton machine registers the Doctor's face as he tries to stop the Gonds smashing the machines. A tentacle like probe comes out of the Kroton machine, finds the Doctor and moves towards him.
The Doctor discovers that the probe doesn't move if he covers his face. It kills a Gond, then retracts. The Krotons order the Gonds to leave once more. The Doctor enters an underground passage, trying to find out more about the make-up of the Kroton machine. Zoe plays with a learning machine, scoring a high mark. Vana begins to recover, recounting a burning light. "Zoe-Gond" is selected by the Krotons; the Doctor takes the learning machine test too, in order that Zoe isn't alone in the Kroton machine. "Doctor-Gond" is accepted too. They enter the Kroton machine, where they are subjected to some machine. Their mental images are transformed into energy which activates a crystal slurry. A figure forms in that slurry tank and they run. The two Krotons watch their escape on a monitor and realise they aren't Gonds and therefore don't destroy them. The Doctor and Zoe get out of the machine; Jamie, though, has entered the machine in order to rescue them. He is subjected to the Kroton scan. They realise he isn't intelligent and say the machine's power will kill him.
The leader Kroton prevents Jamie's destruction because he can tell them about Zoe and the Doctor. Eelek tries to secure help from the scientist Beta. The subordinate Hroton goes to the TARDIS, after the Doctor. Eelek has taken control and wants to fight the Krotons; a debate ensues about their weapons effectiveness against those of the Krotons. Jamie learns that Krotons need to recharge. The Doctor runs tests on the slurry and discovers its chemical composition. He then picks up sulphur. Jamie distracts the Kroton long enough that it cannot give directions to its fellow and this allows the Doctor and Zoe to escape. The Kroton fires on the TARDIS, which dematerialises to another place, because its Hostile Action Displacement System is in operation. The Doctor returns to the city and learns that the Gonds are attempting to knock down three support pillers in the cellers. The Doctor tries to stop this but is buried under collapsing rubble.
The Doctor is unhurt, but many of the Gonds have been. Beta mixes up Sulphuric Acid on the Doctor's instructions. A Kroton comes out and demands that the Gonds hand over the Doctor and Zoe as they are needed to operate the Kroton ship. Jamie manages to get out of the Kroton ship and is told to check on Beta and the Sulphuric acid. The Doctor and Zoe are captured and re-enter the Kroton ship. The Gond leader follows with a bottle and is destroyed. The Krotons were part of a battle fleet; two of their number were destroyed. They require four to fly their ship and thus have been eductaing the Gonds. The Kroton ship is nearly exhausted of energy, which will explode and destroy the planet if not prevented. Zoe pours sulphuric acid from the bottle in the tank of slurry, which the Krotons draw energy from, killing them. Jamie and Beta have poured Sulphuric Acid on the Kroton ship, destroying it. The Gonds are free and must find their own answers. The TARDIS crew leave.
Doctor: Yes....Zoe is something of a genius. It can be very irritating at times.
Zoe: The Doctor is nearly as clever as I am.
Her black PVC mini-skirt and thigh length boots.
The Krotons marks the debut of Robert Holmes who was to write some of the very best stories in the show's history. This though wasn't his best effort. It plods along, the Gonds are a very bland lot and the Krotons are silly looking.
It was the first Troughton story I saw way back as a kid.
The media doesn't change -
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