Doctor Who-The Invisible Enemy

                         Upon redoing the TARDIS main control area, the doctor and Leela find themselves in the year 8000 AD,in the 50th century at a time when humans are heavily exploring their cosmos. The TARDIS is caught up in an unusual energy maelstrom, at which time the doctor and Leela are both struck by an energy beam intimidating from the strange phenomenon into the TARDIS controls. Leela is unaffected. However the doctor has some begins to have unusual symptoms upon landing in a space craft en route for a base on Titan, a moon of Saturn. Upon arriving there the crew,one by one have become infected with a strange virus  that overwhelms the doctor,who all the other people on the ship who have been compromised refer to as "the nucleus". While he attempts to fight off the infection,he and Leela are take to the Bi-Al Foundation Medical Fascility built within an asteroid. There the doctor is treated by Professor Marius,the local expert in extraterrestrial diseases along with his robot dog K-9. 

                With the help of K-9, the doctor requests that Marius use a cloning technique so that miniaturized duplicates of himself and Leela,though with short lifespans, could enter into the doctors infected body  and mind to stop the nucleus from spreading further. While this process is ultimately successful,Marius himself is affected by the nucleus during this time and attempts to send the other compromised staff around him be cloned to enter the doctors body to pursue his clone and stop the nucleus. While the miniaturized clone doctor discovers the sentient macro virus Nucleus and fails to reason with it, it turns out the cells of Leela's body are in fact resistant to the macro virus. While they use this serum to cure Marius,they still have to get it to Titan before the the virus spreads through that base. Upon arriving there,again with the help of K-9, they are able to stop the Nucleus,enlarged by Marius in his cloning machine when he was compromised to blow up the now empty base on Titan with the virus inside. The swarm which originally attacked the doctor is destroyed in the process. When they return to Bi Al to return K-9 Marius asks, as he's returning to Earth if the doctor and Leela can take K-9 with them. And they accept.

               The concept of a macro virus that could effectively influence the mid of a human as a prelude to an invasion was an excellent science fiction concept for the era. Of course this story is important for two reasons. Since the doctor was unconscious in the care of Marius for half of this story, much of the problem solving element goes over to the cybernetic android dog K-9. He proves so popular,especially with Leela, that the doctor even reluctantly agrees to Marius's requests to look after K-9 in his absence. Another crucial thrust of the story is that difference in approach between Leela and the doctor. Interestingly enough Leela's claims that the macro virus created by the Nucleus should be utterly destroyed is as much based in logic as anything the doctor could've thought up. Even though the doctor is adamantly resistant to violence of any sort, he actually takes Leela's advice on this occasion and destroys the Nucleus and its accompanying swarm.