The TARDIS has brought the Doctor and his companions back to the planet Marinus, one century after their previous visit. They are greeted as returning heroes, figures of legend. But something is amiss in the city of Millenius. Everybody lives in peace and harmony...whether they like it or not.

The explanation is obvious. Somebody has reactivated the Conscience Machine and is set on removing free will from Marinus. Somebody - or some old enemies of the Doctor...