The Doctor and Leela land in a jungle on a beautiful planet. Leela eats some of the local fruit, and the Doctor scolds her. Suddenly they hear something large approach. They hide and see a large four-legged animal. The Doctor approaches it and tries to communicate with it, but it runs away. They chase after it, to find it eating some bark from a tree. After this, it starts to speak to them in a strange language. Leela can understand him, but the Doctor cannot.
The Doctor and the creature, named Trok'larr, have exchanged information, using Leela as an interpreter. Trok'larr's people, the P'tarr, live on some nearby islands. The trio are then attacked by soldiers. The Doctor orders Leela to run, and she heads for the P'tarr village. The Doctor is captured by the soldiers, and Trok'larr is killed. The ranking soldier, Lieutenant Cain, regrets the killing and berates Abel, the soldier who killed Trok'larr. They take the Doctor to Colonel Joshua.
The Doctor discovers that the soldiers are humans who had been partially Cyberconverted, then abandoned when the Cybermen were destroyed. They consider themselves to be superior, and Joshua is convinced that humans and Cybermen are meant to be united.
Leela arrives at the P'tarr village and informs them of the attack. Kan'rath tells her that ten of them will prepare for battle, by eating from a certain tree.
The Doctor realises that the vegetation of this planet has coded RNA programmes that allow for learning, which explains why Leela was able to understand the P'tarr language. Joshua hopes to exploit this planet as part of his mission. He eats some of the fruit. The Doctor stops Cain from eating the fruit, but before he can explain why, Leela and the P'tarr attack.
The Doctor tries to call them off, but before he can, Abel is injured. Meanwhile, Joshua has gone to the abandoned Cyberbase and, through the influence of the fruit, sees clearly what he has become. He returns to the Acolyte and kills himself.
Cain promises the Doctor that he and the remaining men will help to keep this planet secret.
- The story has obvious biblical connotations, with Cain and Abel and the concept of the forbidden fruit that reveals the truth.
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