 The Doctor discovers that the volcano is being used by the Pyroviles to convert the human race and conquer Earth . The Doctor realises the volcano will not erupt if the energy converter is running , and tells Donna that the volcanic eruption is a fixed point in time and must always happen . The Doctor and Donna get into an escape pod and together press a lever which overloads the converter and triggers the eruption , killing the Pyroviles and launching the pod clear of the blast . The Doctor and Donna run for the TARDIS and the Doctor coldly leaves Caecilius and his family cowering in their home and dematerialises . Donna tearfully begs him to go back and at least save one person . The Doctor finally relents and goes back for Caecilius and his family . He leaves them on a hill overlooking the destruction , and together with Donna slips out quietly as the family mourns Pompeii . The Doctor comments to Donna that she was right - he does need someone to stop him . 
