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
The Doctor
Louise Jameson
Leela
John Leeson
K-9 Mk. I (voice) / Nucleus (voice)
Michael Sheard
Lowe
Brian Grellis
Safran
Jay Neill
Silvey
Edmund Pegge
Meeker
Frederick Jaeger
Professor Marius
