The Zygon traps the Doctor and Sarah Jane in a decompression chamber while Harry is taken to the aliens' underwater ship.
Quest roles:
Ian Marter(Harry Sullivan), Nicholas Courtney(Brig. Lethbridge-Stewart), John Levene(RSM Benton), John Woodnutt(Seron), Lillias Walker(Sister Lamont), Tony Sibbald(Huckle), Bernard G. High(Corporal), Ronald Gough(Zygon), Robert Russell(The Caber), Keith Ashley(Zygon)