When your enemy is inside your head… sometimes the best cure is to shrink yourself, climb in after it, and shoot it in the face.
Short Synopsis
The Doctor contracts a nasty case of alien mind-virus. The cure? Shrink himself and Leela down to microscopic size to go on an inner-body adventure, complete with giant prawns and a robotic dog. Naturally.
Extended Storyline
It all starts in the year 5000, when a mysterious cloud in space infects a human shuttle crew with something called “the Nucleus of the Swarm.” Think evil space virus… but with delusions of grandeur. The virus spreads fast, jumping hosts like it’s competing in the intergalactic Olympics of contagion.
Unfortunately for the Doctor, he gets zapped while investigating, which means he himself becomes infected. Suddenly, our Time Lord has an alien prawn squatting inside his brain and whispering sweet genocidal nothings about galactic domination. This leaves Leela in the odd position of having to babysit a homicidal Doctor while also fending off possessed humans who now worship the Swarm like it’s some cosmic TikTok star.
To save himself, the Doctor comes up with the sort of solution that only makes sense in Doctor Who: he’ll build clones of himself and Leela, shrink them down to microscopic size, and send them inside his own head to fight the virus. That’s right — the Doctor literally decides to Fantastic Voyage himself. And the BBC actually did it.
Once inside, tiny Doctor and tiny Leela traipse around his brain, battling white blood cells and trying not to think too hard about how ridiculous this all looks. Eventually, they come face-to-face with the Nucleus of the Swarm itself — which, in full form, is revealed to be a giant prawn. A giant. Freaking. Prawn. This is the villain. Someone in the BBC design department looked at a plate of scampi and thought, Yes. Terrifying.
The clones fight valiantly, but they’re on borrowed time — literally. Clones in Doctor Who last about as long as a carton of milk left in the sun. They start to destabilize, leaving real-world Doctor relying on Leela and a friendly robotic invention called K-9.
K-9, designed by Professor Marius (who clearly had too much time on his hands), is a metal dog armed with a laser nose and encyclopedic database. Instantly beloved by Leela, he joins the fight. Together, Leela, the real Doctor, and K-9 manage to lure the giant prawn out of the Doctor’s brain (by growing it to full size — because why not?), where it thrashes about like seafood gone bad before being vaporized.
With the Swarm defeated, the Doctor is cured, the galaxy is safe from a prawn-based dictatorship, and K-9 decides to hitch a ride in the TARDIS as a new companion. Because, really, who wouldn’t want a robotic dog that shoots lasers from its nose?