Episode 1 – The Sea of Death
Episode 2 – The Velvet Web
Episode 3 – The Screaming Jungle
Episode 4 – The Snows of Terror
Episode 5 – Sentence of Death
Episode 6 – The Keys of Marinus
Written by Terry Nation
Directed by John Gorrie
Arriving on a mysterious beach, the travellers initially explore without a care in the world, until they discover the beach is made of glass and the sea is not water but deadly acid. They soon discover they are on the planet Marinus, and meet Arbitan, who lives alone on the island as the keeper of the Conscience of Marinus, a powerful computer that was developed many years ago to maintain law and order across the planet by making its people incapable of committing a crime. However, a splinter group called the Voord, led by the evil Yartek, have found a way to resist its effects, and now plan to invade the island and use the Conscience for their own purposes.
As a defence mechanism, Arbitan has removed four of the fiver operating keys from the Conscience and scattered them across the planet, making it impossible to effect any changes to the way the computer operates. However, Arbitan has managed to discover a way to modify the machine so that Yartek and the Voord will no longer be immune, and so he needs to recover the keys.
Though initially resistant, the Doctor and his companions agree to go on a quest to retrieve the keys, using Arbitan's wrist-mounted travel dials to teleport from place to place on the planet. They first find themselves in the city of Morphoton, where they must free themselves from the hypnotic power of its citizens, then they find themselves trapped in a jungle ruin and besieged by vicious plants. They then have to escape from a murderous hermit in an icy wasteland and defend Ian from a murder charge in the city of Millennius.
Though they successfully find and bring back the keys to the island, Arbitan has been killed by Yartek. The terrorist seems to trick Ian into handing over the keys, but Ian has swapped the final key for an imitation. When Yartek attempts to use the Conscience, the machine explodes, killing him and his Voord followers.
After the incredible success of The Daleks, it was understandable that the production team would again try to capture lightning in a bottle. The Keys of Marinus is the first such attempt. It is also the first attempt at a "quest" serial; a story in which our heroes must complete a specific task each episode, adding up to resolving an overarching objective. By and large, these types of stories do not work as well as ones in which there is a strong single storyline, and The Keys of Marinus is no exception.
There is an episodic feel to the whole thing, with the overarching story not hanging together. It doesn't start off well. The first episode is a virtual retread of the initial moments of The Daleks. This is only the first instance of Terry Nation's unsettling predilection for recycling the same plot devices and concepts over and over and over for Doctor Who. What's even worse is that the design and direction doesn't in any way match those seen in the earlier serial. It's clear this story doesn’t have the budget or the vision that made The Daleks work. The tower set, with its obvious backs drops and shabby corridors, doesn't evoke anything. The Voord are designed in a such a bland and goofy way that not only are they indistinguishable from each other, they're boring.
The individual missions don't work either. Only the first mission, to Morphoton, is interesting. The concept of one character seeing things totally differently than the others is innovative and well-done, but due to having to wrap it all up in one episode, the resolution is rushed and unsatisfying. The jungle episode has some nice moments, but once again, nothing is allowed enough time to breathe and therefore it all feels inconsequential.
The snowbound episode is a little better. At least the story is simple enough for a single episode to not feel rushed or sloppy. But it's also clear that this is the episode where the least amount of money is being spent, so we're left with atrocious stock footage and cheap sets. We're also given a surprisingly dark bad guy in Vasor, who is clearly looking to rape Barbara. He's a truly reprehensible, petty thug, a somewhat interesting change of pace from the more grand villains, but perhaps too dark and intense for the rest of the story.
The last two episodes are the worst of the bunch. The murder mystery/courtroom drama set on Milennius is so blindingly obvious and poorly constructed that virtually no tension or suspense is created. The culprits are easily spotted as soon as they are introduced, so why it takes so long to uncover them is incomprehensible. And it's stretched over two episodes! The final section back at the tower is completely devoid of any real tension, mostly because the Voord and Yartek have no real personality. The way he tries to fool Ian and Susan is so stupid that I don’t know what's worse, that he thinks it will work, or that it seems to fool our heroes even for a second.
Finally, one of the oddest things about this whole serial is that the Doctor is on an adventure to repair a mind-control device. For a character that would eventually become so dedicated to free will and individuality, it's shocking how little fuss is made about this. He puts forward his views at the end, but frankly, the point of the Conscience makes it hard to get behind the whole endeavour. That, combined with the shoddy plotting, is what really sinks this serial.