1 October 1977
Tom Baker (
John Leeson (K9), Michael Sheard (Lowe), Brian Grellis (Safran) [1-2, 4], Edmund Pegge (Meeker) , Jay Neill (Silvey) , Anthony Rowlands (Crewman) , John Leeson (Nucleus Voice)*, Frederick Jaeger (Professor Marius) [2-4], Roy Herrick (Parsons) [2-3], Elizabeth Norman (Marius’ Nurse) [2-3], Nell Curran (Reception Nurse) , Jim McManus (Opthalmologist) [2-4], Roderick Smith (Cruikshank) [2-3], Kenneth Waller (Hedges) , Pat Gorman (A Medic) [3-4], John Scott Martin (Nucleus) [3-4].
|Written by||Bob Baker And Dave Martin|
|Directed by||Derrick Goodwin|
|Produced by||Graham Williams|
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.