The Cybermen take over the base again, apparently to stop the humans using the Z-Bomb, but then the Doctor realises they intend to use it themselves to destroy Earth.
Quest roles:
William Hartnell(The Doctor), Anneke Wills(Polly), Michael Craze(Ben), Peter Hawkins(Dalek (voice)), Roy Skelton(Dalek (voice)), Reg Whitehead(Jarl), Robert Beatty(General Cutler), Dudley Jones(Dyson), David Dodimead(Barclay), Steve Plytas(Wigner), Christopher Matthews(Radar Technician), Harry Brooks(Krang), Ellen Cullen(Geneva Technician), Christopher Dunham(R/T Technician), Callen Angelo(Terry Cutler), Gregg Palmer(Gern)