The one with evil scuba diver aliens.
Written by Terry Nation. This six-episode serial first aired from April 11 to May 16, 1964.
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.
The Doctor and his companions are forced to play along. They team up with Arbitan's daughter Sabetha and her fabulously dressed boyfriend Altos. 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 he's 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.
- All There in the Script: Terry Nation developed more background in his script than was made explicit onscreen in the finished production. The Voord were alien invaders who took advantage of the people of Marinus, rendered vulnerable by the pacifying effects of the Conscience. The Conscience was then deactivated to allow the Marinians to fight the Voord, and over the centuries, the time they spent on Marinus meant that the Voord, too, could now be affected by the machine. Therefore, Arbitan dispatched agents to recover the keys which would reactivate the Conscience and allow him to finally defeat the invaders.
- Attempted Rape: While never stated, it's pretty obvious that Vasor has this in mind for Barbara. Quite a shocking moment for a series like this.
- Awesome Anachronistic Apparel: After spending months in ancient China as Marco Polo's guest, Ian keeps wearing his Chinese clothing for this adventure just for the heck of it.
- Better Manhandle the Murder Weapon: How Ian finds himself accused of murder.
- Big Bad: Yartek. He doesn't appear until the last episode but it is his actions that prompt the Doctor's party to go on their quest to find a way to defeat him.
- Bound and Gagged: Susan in Episode 6.
- Brain in a Jar: The Morpho of Morphoton.
- Brainwashing for the Greater Good: The Conscience. Interestingly, not treated as an inherently bad idea. We get the Doctor's quote at the end, but it had harmlessly kept the peace for ages, and the one human-run justice system we see is a Kangaroo Court where you are guilty until proven innocent. There's just this pesky problem of bad guys trying to hijack the thing.
- Call-Back: Ian is still wearing the costume given to him by Marco Polo in the previous serial.
- Chekhov's Gun: The fake Key.
- The Corpse Stops Here: Probably the earliest example in the series. And Ian is accused of killing him.
- Crapsaccharine World: Morphoton.
- Cutting Back to Reality: The apparently utopian city of Morphoton is actually a Lotus-Eater Machine that keeps its inhabitants happy with illusions. After the brainwashing on Barbara fails, the episode alternates between the illusion that everyone else sees and the grimy reality that Barbara has become aware of. For instance, Susan shows Barbara a beautiful new dress that she's been given, then a cut to Barbara's POV shows her holding a bundle of dirty rags.
- Death World: The planet Marinus. Glass beaches lapped by acid seas. Jungles full of hostile plants and deadly mechanical traps. Frozen wastelands patrolled by packs of man-eating wolves. Bodiless, telepathic slavers. Then there's the WAR....
- Department of Redundancy Department:The Doctor: No. Impossible at this temperature. Besides, it's too warm.
- Also:Barbara: I believe you are under some deep form of deep hypnosis.
- Descending Ceiling: Barbara is nearly done in by a spiked one.
- Dismantled MacGuffin: The Doctor and companions are sent to collect the Keys (scattered throughout the planet Marinus) that control the Conscience Machine, which made everyone peaceful on the planet till it malfunctioned.
- The Evils of Free Will: The villains planned to use the Conscience Machine to control the planet.
- Fakin' MacGuffin: At one location, Ian and Barbara find a fake decoy Key before they find the real one. Ian keeps it, and in the final episode he gives it to Yartek in place of one of the real Keys.
- Faux Affably Evil: The Brains of Morphoton, Vasor.
- Gadget Watches: The teleport bracelets.
- Garden of Evil: The Screaming Jungle. The forest was biologically altered by Darrius in an experiment on speeding up the growth process of flora. The experiments made the flora move. It also seemed to have been made sentient. The flora could kill individuals by constriction. It was also strong, capable of breaking down walls. The flora emitted a loud screaming noise.
- Gotta Catch Them All: The quest for the keys.
- He Knows Too Much: Ayden.
- Also why the Brains of Morphoton decide Barbara must die.
- I Have You Now, My Pretty: Vasor's intentions towards Barbara are never explicitly stated, but they're not exactly difficult to guess.
- I Never Said It Was Poison: Susan is kidnapped and her captor forces her to speak to Barbara over a futuristic alien phone. Later, the kidnapper accidentally lets it slip in conversation that she is aware they have spoken to Susan, even though it was not mentioned. This allows Barbara to realize her guilt.
- Imposter Forgot One Detail: Yarteks disguise as Arbitan is revealed when he has no idea who the guy he supposedly sent to do his work for him is. Not, you know, the fact he looks and sounds more like a Voord than the old man encountered earlier.
- Info Dump: In order to satisfy the demands of BBC executives that the show should still have a primarily educational focus even when taking place away from Earth, Ian and Barbara suddenly drop a rather out-of-place bunch of facts about Egyptian and Mayan pyramids, and the techniques that were used to build them, when they first see the structure that houses the Conscience of Marinus.
- It's Quiet
Too Quiet:Barbara: This is a dead place.Ian: Yes, it's a bit quiet, isn't it?Barbara: That isn't quite what I meant.
- Kangaroo Court: Ian's trial for murder. Under the laws of the alien city where the trial takes place, Ian is guilty until proved innocent; he is even described as "the accused and convicted" before the Doctor has even begun to defend him.
- Let's Split Up, Gang!: Constantly. The gang splits up to search in more places in the same time, and those groups occasionally split to cover more ground. Damsel in Distress-ness ensues.
- This also allowed William Hartnell to go on holiday during the filming of Episodes 3 & 4.
- Lotus-Eater Machine: Morphoton.
- MacGuffin Guardian: One of the keys is in a block of ice and guarded by several warriors who come to life when it is thawed out.
- Malevolent Architecture: In part 3, a building full of death traps houses one of the titular artefacts.
- Mr. Fanservice: Altos, with his clothing covering about as much of his legs as a pair of briefs would.
- Multicultural Alien Planet: The protagonists visit a different area of Marinus in each episode, each with its own inhabitants and culture.
- Names to Run Away From: The Morpho.
- No Name Given: Darrius is never referred to by name in the on-screen dialogue of "The Screaming Jungle"; his name appears only on the closing credits.
- Paper-Thin Disguise: Yartek's attempt to disguise himself as Arbitan is so laughably unconvincing that Ian and Susan come across as complete idiots for believing he was who he claimed to be. It's mitigated a little when Ian reveals that he worked out quickly that "Arbitan" wasn't the real deal and handed him a fake key, though even then it begs to the question as to how exactly Yartek thought his disguise attempt was going to work, other than him presumably just not being able to do come up with a better plan in the time available.
- Patrick Stewart Speech: The Doctor delivers the page quote.
- People in Rubber Suits: The Voords. Possibly. Perhaps the suits have fused with their actual bodies.
- Rope Bridge: A rope bridge is used to trap the protagonists on the wrong side of a chasm inside a spacious ice-cave. They put it back together with the help of stalactites (or were they stalagmites?) even though they clearly could have just jumped over the chasm.
- "Shaggy Dog" Story: Somewhat unusually, it's signposted at the end of the first episode that the Doctor's and his companions' quest will turn out to be this, when Arbitan is assassinated by one of the Voords. Though it does end up being a downplayed example, as the Conscience of Marinus self-destructs at the end of the story, but ends up taking the Voords with it.
- Single-Biome Planet: Defied. Each episode takes place in a different environment within Marinus.
- Take Our Word for It: In Morphoton, the Doctor is offered a laboratory fitted out with an impressive array of advanced scientific equipment — which the audience never sees. By this point, Morphoton's Lotus-Eater Machine nature has been revealed to the audience, so the scene is depicted by Cutting Back to Reality where the Doctor is standing in a bare room rhapsodizing over things that aren't really there.
- Trouble Magnet Gambit: Vasor is the former Trope Namer.
- Villainous Rescue: Vasor takes Susan hostage, but the warriors from the ice kill him as they hack their way through the door.
- Visible Boom Mic: In "The Velvet Web", as the group are about to sleep in Morphoton, a boom microphone appears at the top of the shot; William Russell can clearly be seen glancing at it briefly.
- Wacky Wayside Tribe: The serial is entirely made of this - the characters have to collect Plot Coupons from various locations on the planet, each of which has a different culture and threat. There's a Lotus-Eater Machine world that only Barbara can see through, a murder investigation world where the Doctor is a lawyer and has to use The Perry Mason Method to save Ian after he picked up the knife, an ice world where they have to fight ancient guardians and so on.
- The X of Y: Most of the individual episodes count as well, with the 1st, 4th, 5th and 6th being called The Sea of Death, The Snows of Terror, Sentence of Death and The Keys of Marinus respectively.