Every now and then, the Doctor likes to go on video game-style quests. His first time doing this was in The Keys of Marinus. Marinus is a planet where morality is entirely regulated by a machine that eliminates all evil thoughts from people’s heads…except for Yartek and his alien minions the Voord, who may or may not be wearing Venom-like symbiotes. Arbitan, keeper of the Conscience, scattered the keys to the machine across the planet to keep Yartek from using the machine for his own purposes. Being the nice keeper of morality that he is, he holds the TARDIS hostage so the Doctor and his companions will find the keys.
This was a rather long, convoluted serial, comprising as it did several quest-style storylines. In fact, they discover several places where everyone is just as horrible. You have Morphoton, where everyone is hypnotized into believing they live in utter luxury (the Doctor is promised a highly advanced lab where he can science to his heart’s content); Barbara is able to break free and find Arbitan’s missing daughter, and the key. The adventurers split up after this, finding a jungle that affects telepaths (such as Susan) and also where plants want to kill you (such as Audrey II), an icy land where Vasor creates a trope, and Millenius, where you can’t help but wonder if the writers were just plain making fun of the British court system at that point.
I can’t help but feel this was just the way to show off the various locations the creators can come up with. Some of the special effects were remarkably well done for the time.
Some...not so much.
Best. Special effect. EVER.