A spaceship near the outer planets of the solar system, cruising on autopilot, is possessed by a strange virus—first the ship’s computer and then the human crew. They take over Titan Base as a breeding ground. Station Manager Lowe (Michael Sheard) sends out a distress call. The TARDIS is affected by the same space virus. The Doctor and Leela hear the distress call and investigate. The Doctor is overcome by infection and is chosen to be the host of the nucleus of the swarm. Leela is not infected. She is rejected and slated to be killed. The Doctor manages to break the infection and tells Leela how to get to the nearest medical centre. Lowe accompanies them. They do not know that Lowe is already infected.
At the Medical Station, Professor Marius (Frederick Jaeger) introduces them to his robot dog K9. Marius is baffled by the virus, and Lowe is infecting everyone. The Doctor and Leela create short-lived clones of themselves, which are shrunk to microscopic size and injected into the Doctor. After a hazardous trip through the Doctor’s mind, their clones are separated, and the Doctor’s clone reaches the nucleus. Professor Marius retrieves something from the Doctor’s tear duct and enlarges it, but it’s not the Doctor, it’s the nucleus, which is enlarged into a kind of space-prawn. Still, the Doctor is cured.
The nucleus and the infected staff depart for Titan Base to spawn. The Doctor is cured because of Leela’s immunity. A replica of the immunity is passed on to Professor Marius to use as a cure, as the Doctor, Leela, and K9 head for Titan base. They fight off the infected humans. But they need to kill the infection. The Doctor rigs a gun to fire into a cloud of oxygen. The blaster fires, igniting the oxygen in the methane atmosphere, destroying the swarm and the whole base. Professor Marius is due to return to Earth soon, so he offers K9 to the Doctor and Leela.
Obviously, the story was inspired by the Fantastic Voyage movie; too bad it didn’t have the 20th Century Fox budget. The TARDIS console set had been rebuilt because the older one, with the wood paneling, had begun to warp. A set of the moon from Space 1999 was used as Titan. Louise Jameson was right-handed but chose to write with her left hand to make Leela seem awkward at the new skill. The serial itself was not loved, but K9 was. In the final analysis, K9 was the only thing in the serial that actually worked. It was decided to use the little robot as an on-board companion to offset the cost of building it, and the character became instantly popular, mostly because of John Leeson, who provided the voice. Tom Baker found K9 difficult because he was constantly forced to get down on his hands and knees to act with it, and that was painful, but the cast and crew simply adored John Leeson. In rehearsals, the machine was usually being repaired, and he would get down on his hands and knees and act like a dog, keeping everyone in stitches.