The Invisible Enemy - Part One
A spaceship arrives at Titan Base after passing through a mysterious cloud. The three human crew members have been infected and are now possessed by whatever was in the cloud. They are supposed to change places with three others who are waiting for them on Titan but the three from the shuttle kill their replacements.
The three possessed men go after Lowe, the station supervisor. He sees that something is wrong and sends out a distress signal. The Doctor and Leela receive the distress signal and head for Titan Base. They pass through the same mysterious cloud. The Doctor is infected but doesn't know it. Leela senses that something is wrong but the Doctor won't listen to her.
Lowe tries to arrest the three possessed me. He shoots at them, killing one of them, when they don't drop their weapons. The Doctor and Leela find the body. but no one else. The Doctor tells Leela to stay put but she goes exploring. She finds Lowe almost dead. She revives him and asks him what's going on.