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, Louise Jameson, John Leeson, Michael Sheard, Brian Grellis, Jay Neill,

Genre:Science Fiction, Drama, Adventure,

country:United Kingdom,

Production:BBC,