Ep 5. The Invisible Enemy: Part 1

The Doctor and Leela find themselves on a deserted spacecraft drifting in space. Upon exploring the ship, they discover that it is infested with a virus that turns humans into zombie-like creatures. The culprit of this virus is a parasitic alien race known as the Swarm, who seek to take over the universe by merging with their host's consciousness. The Doctor and Leela must find a way to stop the virus from spreading and prevent the Swarm from taking over the universe. The Doctor constructs a miniature version of himself and sends it into his own brain to combat the virus, while Leela must take on the Swarm's leader on her own. Along the way, they encounter a sentient computer named K-9, who joins them on their journey and proves to be a valuable asset in their fight against the Swarm. The episode ends with the Doctor being taken over by the virus, setting the stage for the next installment in the two-part story.