Doctor Who/Recap/S1/E05 The Keys of Marinus

"I don't believe that man was made to be controlled by machines. Machines can make laws but they cannot preserve justice. Only human beings can do that."

—The Doctor

Back with science-based action this time, as the TARDIS arrives on Marinus, on an island of glass in a sea of acid. It turns out this island is the location of the "Conscience of Marinus", a mind-controlling machine that has been used for centuries to keep the peace. However, the villainous Yartek and his Voord warriors wanted to capture the machine and use it for evil, so the four eponymous Keys of Marinus without which the machine wouldn't function were scattered over the planet in secret locations.

Now Arbitan, the "Keeper of the Conscience" has rigged the machine so the Voords can't use it and the Doctor and companions must go and retrieve the Keys of Marinus so it can be switched back on again.

After adventures in a series of inventive locations including a building under attack by ambulatory plants and a city in which Ian is accused of murder and finds that here, you are guilty until proven innocent, the travellers return with the Keys, only to find that Yartek has killed Arbitan. Yartek forces them to hand over the keys, but Ian passes him a fake one and the machine blows up in his face, killing him and the other Voords.

Watch it here.^{[dead link]}

Tropes

The Doctor: No. Impossible at this temperature. Besides, it's too warm.
...
Barbara: I believe you are under some deep form of deep hypnosis.

Descending Ceiling: Barbara is nearly done in by a spiked one.
Fakin' MacGuffin
He Knows Too Much: Ayden
I Never Said It Was Poison: How one of the conspirators in episodes 5 and 6 slips up.
Kangaroo Court
Lotus Eater Machine: Morphoton
Memetic Mutation: Yartek, LEADER OF THE ALIEN VOORD! is a classic one, stemming from a description of the story in tie-in material.
Multicultural Alien Planet
Shifting the Burden of Proof: Jurisprudence in Millennius.
Special Effects Failure: In the first episode a villain falls to their death and its very obviously not a real person but looks like a figurine of some sort.
Trouble Magnet Gambit: Vasor
The X of Y