The past few episodes, the Doctor has been in the secondary control room. This time he moves back to the original one. Hearing a distress call, the Doctor sets the co-ordinates to help if he can. As he's checking a few of the controls, the Tardis runs into a storm. The Tardis is infected with a virus that is passed along to the Doctor. Leela is also at the controls, but the virus rejects her.
The Doctor is responding to the distress call of Titan base. A ship dispatched to relief the onbase crew, also passes through the same storm as the Tardis. Upon arriving at Titan base, they kill the crew, though Lowe the station supervises. The infected personal cancel the distress call and are made aware the Doctor is arriving and carries the main host of the infection. The Doctor is initially unaware of this, but Leela senses danger.
Leela manages to kill one of the two remaining infected crew, but not before he infects Lowe. One indication of the contagion is a scaling around the eyes, so he dons a pair of space goggles and claims he was blasted in the eye. He offers to take the Doctor and Leela to a nearby medical facility to help the Doctor who is now showing signs he's infected. On the medical asteroid, they meet Marius and his companion K-9.
With Marius and K-9's help, the Doctor realizes the only way to destroy the infection within him is to go inside his body and destroy it face to face. Marius and K-9 are familiar with ways of cloning both the Doctor and Leela, but the clones only have a life span of 10 minutes. The Doctor believes this is enough time and has Marius clone him, then using an instrument from the Tardis, they are shrunken down and injected inside the Doctor.
Though only having a short amount of time, the Doctor and Leela find the virus but their time has worn out. The virus knows of the Doctors 'escape route' and escapes through the Doctor's tear ducts. Marius (now infected) thinks it's the Doctor and unshrinks him to interrogate. The Virus, now man sized, plans to spawn and infect everyone in the 'macro universe'. Heading back to the asteroid, one of the infected personnel has prepared for the spawning, but the Doctor is successful in destroying the virus and all the spawn.
PLANET OF ORIGIN: Bi-Al Foundation, Asteroid K4067; Year 5080.
REASON FOR JOINING: Marius explains to the Doctor he's heading back home and won't be able to bring K-9, he offers K-9 to the Doctor. The Doctor and K-9 are eager to team up.
NEXT: Time for the Invasion