The Doctor realises that Zeta Minor lies on the boundary between the universes of matter and anti-matter and something on the planet won't let the Morestrans leave.
Quest roles:
Terry Walsh(Doran (uncredited)), Michael Wisher(Ranjit (voice, uncredited)), Prentis Hancock(Captain), Louis Mahoney(Ponti), Ewen Solon(Vishinsky), Frederick Jaeger(Professor Marius), Graham Weston(De Haan), Haydn Wood(O'Hara)