On the planet Vortis, the Doctor and his companions aid the butterfly-like Menoptra in their battle against the parasitic Animus and its mind-controlled minions, the ant-like Zarbi.
William Hartnell
as The Doctor
Jacqueline Hill
as Barbara Wright
William Russell
as Ian Chesterton
Maureen O'Brien
as Vicki
Catherine Fleming
as Animus (voice)
Roslyn de Winter
as Vrestin
Arne Gordon
as Hrostar
Arthur Blake
as Hrhoonda
Jolyon Booth
as Hlynia
Jocelyn Birdsall
as Hlynia
Martin Jarvis
as Hillo
Ian Thompson
as Hetra
