Parts: 4
Writers: Bob Baker and Dave Martin
Airdates: 1 October 1977 - 22 October 1977
Production Code: 4T

Doctor: fourth
Companions: Leela, K-9 mark one

Creature Feature: Nucleus

Story: In Earth's future, when mankind has begun to explore the stars and spread through space in greater numbers than ever before, a ship carrying the relief crew for Titan Base encounters a cloud-like organism in space. Energy crackles between the cloud and the ship; and onboard, the computer intones, "Contact has been made."

When the crew land at Titan Base, they immediately proceed to the lounge, where the outgoing base members are celebrating their imminent departure. But instead of joining in the celebration, the replacement crew gun down the staff. Only one member remains - the station supervisor, Lowe, who realizes something has gone terribly wrong and sends out a mayday.

Meanwhile, when the TARDIS materializes nearby, it receives Lowe's distress call. As the Doctor sets the co-ordinates for Titan Base, the TARDIS passes through the cloud. Energy crackles between the console and the Doctor's forehead, and he collapses; but Leela is unaffected. When she brings the Doctor round, they go to investigate the base...but something has managed to infiltrate the Doctor's mind, and it orders him to kill Leela - for she is a reject, and therefore a danger to the Purpose. The Doctor has become host to a viral intelligence, and unless he can expel it from his mind, it will infect all of humanity throughout time and space...