The Invisible Enemy is based on a huge equivocation.
Episode One is explicitly about possession. The Swarm inhabits and controls people's minds, passing from host to host by zapping them in the eyes with Force Lightening. It jumps into the TARDIS memory banks (because they are so advanced and clever) and then into the Doctor's Mind (because he is the cleverest person in the room). Later in the story, it possesses K-9, a robot, who is arguably capable of thinking but who certainly does not have an organic brain. But by Episode Four, the mind-virus has become a physical, prawn-like entity which infects the actual grey matter inside the Doctor's cranium, and which can (in principle) be resisted by antibodies in the blood-stream.
There is a kind of ghost story in which the ghost is real: the hero is a priest or mystic who knows how to exorcise it or lay it to rest. But there is a more secular kind of ghost story in which Harry Houdini, or the Rev. Tilney, or indeed Fred and Velma exorcise the ghost by explaining it away. It is still defeated; but it is defeated by the power of rationalism rather than the power of God. The demonic hound is turned into a perfectly ordinary german shepherd smeared with phosphorescent paint; the Mummy turns out to be an unscrupulous archeologist who wanted to steal a gem from the museum. And he would have gotten away with it too, if not for...
The Invisible Enemy may make slightly more sense if we try to read it in that way. The spiritual force which has possessed the minds of the astronauts, the station manager, and the Doctor is drawn out of the realm of ideas into the real world. It escapes from the surreal mindscape of the Doctor's brain and manifests in a mainstream science fiction world of corridors and airlocks. It is transformed from a force which might have made the Doctor murder Leela into one more megalomaniac alien who wants to rule the universe. ("For no adequately explored reason" as the next script editor but one would have put it.)
So perhaps the inadequacy of the Prawn Special Effect is not unintentional. The Nucleus is supposed to be ridiculous: Science! has rendered it non-threatening. The Doctor calls it "a pathetic crustacean". (Tom Baker reportedly sang the theme song from "Prawn Free" whenever it came on set.) Drawn out of the realm of thought into the realm of bodies, it can be defeated.
Not, in the end, with an antidote; but with a very large bomb.
We start with three men in space suits. They would not have looked out of place in any Patrick Troughton story. We have a space ship that looks distinctly like a space shuttle. The Space Shuttle was much in people's minds: the first testing flights, off the back of a Jumbo Jet, had taken place the previous August. They have the kind of conversation that these kinds of space-men always have in this kind of story, gently telling the viewer that space travel is not glamorous and heroic but a routine slog. ("Look, I qualified for exploration eight years ago, and what am I? Glorified garage attendant on some planetary filling station.")
And then a Thing happens.
The spacemen get possessed by an alien force. They start talking in monotones about how "contact has been made" and "this planet will be suitable for our purpose". We can tell they have been possessed because they have sinister white fur on their skins, like out of date blackberries.
This is standard issue Doctor Who, and it is not badly done. The music is extreme and melodramatic and the acting is completely over the top. Innocent lines like: "I will stay with them to guard the nucleus and destroy the reject" become "to GUARD the nucleus, and to...des TROY...... the reject", jabbing one finger in the air and then making a fist. Alarms go off, maydays are issued; we see extreme close ups of trigger fingers on ray-guns.
We know how this plays out. The Doctor arrives on the base, finds it besieged by the possessed astronauts; and begins a race against time to find the cure while more and more of the humans succumb to the infection.
But Bob Baker and Dave Martin, to their credit, throw away the rule book. The Invisible Enemy does not proceed according to formula. In Scene One, the space shuttle gets zapped by the alien force, and the three space men get possessed. In Scene Two, the TARDIS itself gets zapped, and the mysterious alien force takes over the Doctor. He resists it by putting himself into a trance.
This is not merely quite a good twist: it's a mild subversion of the whole idea behind Doctor Who. Or, if you are certain kind of 1970s Whovian, a terrible debunking of its magic. The Doctor is meant to function as a deus ex machina who comes along and sorts everything out. This time he's directly threatened. Indeed, he himself is the threat.
The question is not "Can the Doctor save the space-men?" but "Who can save the Doctor?"
The answer, being "The Doctor", obviously enough.
Horror of Fang Rock felt like the continuation of the Hinchcliffe era by other means. Invisible Enemy feels like a dissonant change of tone. The old silver TARDIS control room is back, and Leela feels incongruous in it: not merely a savage on a space ship, but an imposter on a set created in a different era. She is not the same character she was even a week ago. She is back in her leather bikini, but she spends a good chunk of the first episode wearing the Doctor's hat: at one point she appears to be chewing on his scarf. (We hardly notice the oddness of the Doctor's costume any more. They are on a spaceship. He is in outdoor clothes; Leela is dressed for the beach.) In Fang Rock, the Doctor and Leela were two grown up, mutually respecting characters who had conversations with each other. Invisible Enemy is full of this kind of thing:
"Saint Elmo's fire. It happens at sea."
"Yes, it causes a sort of halo effect round the masts of ships."
"Why do you keep repeating everything I say? You're not a parrot, are you?"
"Yes. A parrot's a bird that repeats things. Move over."
This hardly qualifies as dialogue: it is a bit of panto, a cross-talk routine of this kind Basil Brush had with Mr Roy on a weekly basis. Leela is the comedy primitive; the Doctor is the comedy smart-alec. No longer characters: more like Doctor and Leela off the back of a box of Weetabix.
In order to cure the Doctor of his possession, it is necessary to get from Titan to a Centre for Alien Biomorphology in the asteroid belt. Leela suggests that they "take the TARDIS". Lowe, the supervisor of the crew of the Titan Base, himself possessed by the virus, tells them where Biomorphology Centre is; and the Doctor comes out of his trance to tell them the coordinates. Leela, who can't reliably write her own name, programmes them in.
In the past, the TARDIS was de facto and sometimes canonically involatile. You are safe once you are inside it: but you can't go back to it until you have finished this month's adventure. Now the Doctor can be zapped in his own control room by what is, frankly, a distinctly second division bad guy. And the TARDIS can be used to give people a lift from one part of the story to another, and a not particularly clever companion can operate it. Lowe's TARDIS trip takes place off-stage. We don't find out if he was surprised by the TARDIS' internal dimensions. And in a way, why should we? We take the TARDIS -- and the Doctor -- for granted; so why shouldn't the non-player characters?
But a TV show where the Doctor is a clever alien with an impressive vehicle is a lot different from one in which the Doctor is a mysterious wanderer with a wondrous Ship. Horror of Fang Rock would have been over very quickly if it had occurred to the Doctor to use the TARDIS as a taxi to ferry everyone back to the main land.
Once they arrive on the Centre For Alien Morphology (which turns out to be a space hospital) the story, and indeed the series, pretty much turns into a cartoon. Was I the only person who saw the hollowed out asteroid, with a Red Cross emblazoned on the surface, and thought we'd moved to the Clanger's moon Oliver Postgate could have supplied a better class of prawn.
We meet some nurses in strange green uniforms and eye make up. We notice some mis-spelt notices. And we encounter a scientist with a beard and a silly accent, who keep taking ticker tape out of the mouth of a robot dog.
Leela was, we are often told "something for the dads": K-9 was arguably something for the kids. We have shifted from a world where astronauts talk like truckers and drink liquor in the mess to a world where scientists with beards and tweed jackets talk pseudo science in Dr Zarkov accents. Nerdy kids like big words: I can remember driving my parents mad saying "deactivate it" instead of "turn it off". But we have had Hal and we are weeks away from Threepio and Zen and Marvin: a robot who says "affirmative" and "negative" instead of yes and no feels retro. It is never quite clear if K-9 is mainly a toy, a pet, or if Doctor Zarkov actually needs a portable computer with a death ray in its nose and has made it dog shaped on a whim. I felt, and still feel, that a group of Daleks sliding around the studio floor in formation look cool and alien; but K-9 just comes across as a shopping trolly or a motorised wheel chair.
Basil Brush had a battery operated toy dog called Ticker that would interrupt Mr Roy's stories by doing back-flips and barking. ("Quiet Ticker. Shut your little bone-shoot.")
According to K-9, what the Doctor has been possessed by is an "unidentified viral type infection with noetic characteristics" which is "at present seated in the mind-brain interface and therefore having no ascertainable mass or structure".
Nous is "mind"; noetikos is "intellectual"; "noetics" means the philosophy of mind. So, "a virus with noetic characteristics" might simply mean "a micro-organism which can think for itself". But this micro-organism exists in the place where the material universe (the brain) interfaces with the immaterial (the mind) and therefore has no weight or form. So I think that by "virus with noetic characteristics" the comic relief robot means "a microscopic pathogen which shares some features in common with thoughts". Thoughts, after all, have no mass or structure. Arguably.
The idea that the mind and the brain are different things is called Cartesian Dualism, after Descartes, who is also to blame for Calculas. Arguably. The question Cartesian Dualism can't answer (arguably) is how the supposedly perfect, incorporeal "mind" interacts with that lump of matter we call the brain. Descartes' answer was "through the pineal gland" which presumably went down better in the seventeenth century that it does today.
If the brain and the mind are separate, then the idea of a mind/brain interface is pretty much inconceivable. If the mind is simply the word we use for some of the things the brain does, then the idea of a mind/brain interface is pretty much without meaning. And it is in this inconceivable, meaningless place that the virus-with-thought-like-qualities resides. The Doctor is being controlled from a place which does not exist by a thing which does not exist.
What did Bob Baker and Dave Martin think they were doing? Were they making a serious attempt to use a children's TV show to talk about the mind/body problem. Tea-time philosophy for tots, as it were? Are they dimly trying to draw an analogy between the virus and quantum mechanics? A photon is in some sense a wave and a particle simultaneously: and the Nucleus is in some sense both an organism and an idea?
Or were they just filling the air with meaningless gibberish?