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.

• BBC
• Tom Baker
• Louise Jameson
• John Leeson
• Michael Sheard
• Brian Grellis
• Jay Neill
• Edmund Pegge
• Frederick Jaeger