It is considered a cult film, with its mix of mythology, a bizarre, sprawling plot filled with twists and incongruities, wide-ranging satirical and allegorical stabs, dated 1970s aesthetic, and being filmed in an almost perpetual soft-focus haze. Boorman's studio, 20th Century Fox, was dismayed at the uncommercial mess he presented to them and forced the addition of a misleading prologue and trailers that were at odds with the film.
Warning: Spoilers follow
Plot
In a future post-apocalypse Earth (2293) the human population is divided into the immortal 'Eternals' and 'Brutals'. The Brutals live in a wasteland, growing food for the Eternals, who live apart in the Vortex leading a luxurious but apathetic existence in the grounds of a country estate. The connection between the two groups is through Brutal Exterminators, who kill and terrorize other Brutals at the orders of a huge flying stone head called Zardoz, which supplies them with weapons in exchange for the food they collect. Zed (Connery) is a Brutal Exterminator. He hides aboard Zardoz during one trip, "temporarily" killing its operator-creator Arthur Frayn (Niall Buggy).
Arriving in the Vortex Zed meets two Eternals, Consuella (Charlotte Rampling) and May (Sara Kestleman), and is experimented upon by one of them before being made to work for another Eternal, a troublemaker called Friend (John Alderton). The Eternals are overseen and protected from death by an AI called the Tabernacle (a large crystal mass in appearance) and have problems themselves, falling into catatonia through an odd illness or being deliberately aged into senility for violating the rules. Zed discovers an old book, the "Wizard of Oz", and he realizes the origin of the name Zardoz. It is then revealed that Zed was created by Frayn to destroy the Vortex and restore death to the bored Eternals. Zed finds a flaw in the Tabernacle and destroys it, then leads the Brutals in besieging the Eternals and finishes them bloodily before the film ends with another twist to create a (relatively) happy ending.