With factions vying for control of the ship, the Vervoids are found to be responsible for the many disappearances. Only the Doctor sees the fundamental but missed point as to why they can't be reasoned with or why they can't be allowed to arrive on Earth.
Quest roles:
Lynda Bellingham(The Inquisitor), Michael Jayston(The Valeyard), Arthur Hewlett(Kimber), Tony Scoggo(Grenville), Honor Blackman(Professor Lasky), Michael Craig(Commodore), Malcolm Tierney(Doland), Simon Slater(Edwardes), Yolande Palfrey(Janet)