The Keys of Marinus

On a small island with a glass beach, surrounded by an acid sea, on the planet Marinus, stands a tower with many secret entrances. Within the tower is Arbitan, Keeper of the Conscience of Marinus, a vast computer developed two millennia earlier as a vast justice machine which kept law and order across the entire planet. For seven hundred years, the Conscience was absolute, radiating its power across the planet Marinus, and eliminating all thought of evil. But then Yartek, leader of the alien Voord, worked out how to resist its impulses.
　　When The Doctor and his companions Barbara Wright, Ian Chesterton and Susan Foreman arrive on the island, they are brought into the tower to an audience with Arbitan, who explains that the society of Marinus is in danger. Several submersibles containing Voord, humanoid creatures protected by amphibian-like black rubber wet suits, have washed up on the beach. Inspired by Yartek, the Voord are seeking to enter the tower and take control of the Conscience.
　　Arbitan explains that the Conscience has now been upgraded sufficiently to control the Voord again, but needs to be activated. Years earlier Arbitan had prevented the Conscience from falling into Voord control by separating the five Keys needed to regulate it. The five keys are in different locations - one is in Arbitan's possession, but the other four are scattered over Marinus. The keys can only be found by following directions pre-set into travel dials, watch-like devices with the power to transport the wearer across the planet to the correct locations. Arbitan asks that the Doctor and his friends help him fend off the Voord by gathering the keys together. Others have tried to accomplish this task - even Arbitan's own daughter - but none have returned to the tower.
　　The Doctor refuses Arbitan's request, but is unable to access the TARDIS due to a force field Arbitan places around the ship. And so the Doctor and his companions are coerced to aid Arbitan. As the four teleport away from the tower using the travel dials, Arbitan is attacked and stabbed to death by a Voord that has secretly gained access to the tower.
　　The first location visited by the travellers is the City of Morphoton. The seemingly advanced and pacifist inhabitants impress the travellers with the luxuries, advances and aesthetics of the city. But all is not as it seems. Barbara is the first to realize the truth when a hypnotic disc intended to make her mind receptive to the hypnotic pulses slips off her forehead, causing her to realise that Morphoton is actually a place of dirt and squalor rather than beauty and luxury. Unknown to the Doctor and crew, Morphoton is governed by four brain creatures with hideous eyes on stalks who, having outgrown their bodies, live in large bell jars and communicate through their life-support machines. The Brains of Morphoton use hypnosis to control the human population, and the entire City is subjugated to their will.
　　Once the Brains realize Barbara has seen the truth and is thus impervious to their hypnotic control, they order that she be killed. Barbara escapes and hides in the city, there making contact with the slave girl Sabetha, who has been blamed for Barbara's awakening and sentenced to death. Barbara deduces Sabetha is Arbitan’s missing daughter, and sees Sabetha wears one of the Keys about her neck. Barbara helps break Sabetha's conditioning, and together they escape and destroy the jars and equipment protecting the Brains. With their life-support ruined, the Brains die, and all the human subjects of the city are freed. Another slave called Altos remembers he too was s