On the Earth colony world Terra Alpha, Helen A seeks to eliminate all forms of unhappiness one killjoy at a time, it's up to The Doctor and Ace to put a stop to the overbearing joy, one sweet step at a time...
Sylvester McCoy , Sophie Aldred , Sheila Hancock , Ronald Fraser , Georgina Hale , Rachel Bell , Harold Innocent , John Normington , Lesley Dunlop , Tim Barker
