The Earth colony Terra Alpha, the future. With the aid of her sweet-natured executioner, Helen A, ruler of the planet, has ensured the population is happy - on pain of death! The Doctor and Ace decide to fight for the right to be miserable.

The Doctor and Earl manage to escape from the Kandy Man and join up with the planet's natives, the Pipe People, enlisting their help to rescue Ace.

As the Doctor leads the Pipe People towards reclaiming their world, Helen A faces betrayal from those closest to her.