Zardoz

In the distant future Earth is divided into two camps, the barely civilized group and the overly civilized one with mental powers.  One day a barbarian, Zed, stows away in the giant head that supplies the barbarians with munitions and food and that his people worship.  When the head returns to its station in a lush city, the barbarian finds himself surrounded by the telekinetic ruling class of "Homo Eternals". Zed finds out that the Eternals control the head and are using it to rule over the barbarian people.  A plague is attacking the Eternals which causes the victims to have a lack of interest in life and become nearly catatonic.  After being captured by the Eternals using there telekinetic powers, the group decides to study Zed.  Zed's capture causes a great upheaval as the Eternal's lives and the tenuous balance in their world is seriously threatened.