The Doctor and the Brigadier begin to realise who the Zygon imposters among them are while Sarah Jane discovers a secret tunnel to the underwater ship.
Quest roles:
Ian Marter(Harry Sullivan), Nicholas Courtney(Brig. Lethbridge-Stewart), Angus Lennie(Storr), John Levene(Sergeant Benton), John Woodnutt(Seron), Lillias Walker(Sister Lamont), Ronald Gough(Zygon), Peter Symonds(Soldier), Robert Russell(The Caber), Keith Ashley(Dalek Operator)