The Nimon has promised to restore the Skonnan Empire to its former glory. But first it demands sacrifice – youths and maidens from the peaceful planet Aneth.

The Tardis collides with the space ship delivering the victims, and the captured Romana is condemned to be sacrificed to the Nimon.

Aided by the faithful K9, the Doctor goes to the rescue. In the heart of the maze he confronts the Nimon and uncovers a terrifying plot to enslave the galaxy.