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.

Cast

Tom Baker

The Doctor

Louise Jameson

Leela

John Leeson

K-9 Mk. I (voice) / Nucleus (voice)

Michael Sheard

Lowe

Brian Grellis

Safran

Edmund Pegge

Meeker

Frederick Jaeger

Professor Marius

Jay Neill

Silvey