A three-man rocket crew are nearly done with their mission to Titan Base until a course change puts the rocket in the path of a strange cloud in space. By the time they arrive, they have come under the control of a sentient virus which threatens the galaxy. When the TARDIS picks up an emergency message, it flies into the cloud, infecting the Doctor. To save himself and others, he must undertake a dangerous journey.

Tom Baker
as The Doctor

Louise Jameson
as Leela

John Leeson
as K-9 Mk. I (voice) / Nucleus (voice)

Michael Sheard
as Lowe

Brian Grellis
as Safran

Jay Neill
as Silvey

Edmund Pegge
as Meeker

Frederick Jaeger
as Professor Marius
CinemaSerf
Hmmm. This one struggles to get going and has possibly the most un-menacing of antagonists I think I ever saw our time travellers have to face. The "Doctor" (Tom Baker) and "Leela" (Louise Jameson) respond to an emergency message from "Titan" and find when they get there that the crew have been infe...