Stanley Donen's otherwise witty and diverting science-fiction thriller "Saturn 3," a parable of jealousy set on a remote, futuristic Eden suddenly contaminated by insane lust, suffers desperately for the lack of an epilogue.

As a result, an hour and a half of tense, funny sexual melodrama is squashed flat by a dud of a fadeout.

Originally fronting as the executive producer, Donen took over the production after the death of his friend and colleague John Barry, the noted British art director ("A Clockwise Orange," "The Little Prince," "Star Wars" and "Superman," among other credits), who was to make his directing debut on "Saturn 3." Barry evidently suggested the story outline, which was turned into a screenplay by the young novelist Martin Amis.

Farrah Fawcett, accorded top billing and used rather cleverly as an ingenuous beauty in distress, embodies and extraterrestrial Eve named Alex who cohabits with Kirk Douglas, an aging but still vigorous agronomist named Adam, on an experimental, subterranean agricultural station on a moon of Saturn. Their romantic-scientific idyll is disrupted by the arrival of a rare visitor, Harvey Keitel as a technician from distant headquarters who calls himself Captain James.

Encountering Alex for the first time, the newcommer gets right down to essentials. "You have a great body," James remarks. "May I use it?" When Alex protests her fidelity to Adam, James gets very put out: "You're for his exclusive use?" he sneers. "That's penally unsocial on Earth."

Nevertheless, Alex is an old-fashioned girl who has never set foot on the decaying Earth of a century or so hence, and considers it unthinkable to betray the man she loves. Seething at her refusal, the presumptuous James resorts to underhanded methods. The audience has already seen how ruthless he can be: In the opening sequence, he gruesomely disposes of the real Captain James and takes over his mission to Saturn 3.

Adam and Alex learn soon enough that they're got a psychopath as well as a nuisance on their hands. As the agent of his spite, James uses a robot called Hector, the prototype of a new improved line, "the demigod series," ostensibly designed to assist Adam and Alex and the older robots increase productivity down on the farm.

James assembles Hector, a transparent mechanical brute traversed by intricate tubular "veins" and eerily distinguished by glowing vacuum-tube "eyes" which swivel at the end of a long, retractible steel "neck." Expressive, resourceful and powerfully intimidating, Hector is a wonderful high-tech menace, one of the best mechanical monsters ever to stalk the screen.

In contrast to the inscrutable monster of "Alien," Hector is stylishly tangible. We see how his ingenious anatomy functions and how his lightning reflexes might be diverted from useful to terrifying employment.

Psychologically, he never stands a chance. Programmed by the demented James, he inherits a pathological hatred for Adam and lust for Alex along with his intended skills as a superhuman farmhand. In fact, Hector absorbs so much of James' mentality by telepathic suggestion that he becomes a deadly threat to his programmer as well as his hosts.

At peak melodramatic efficiency "Saturn 3" combines elegant pictorial design with explosive cat-and-mouse tension. Donen's graceful touch is evident in the crisp, fluid composition and the general enhancement of suspense and excitement through deft visual timing and embellishment. I loved the reprise of his characteristic low-angle tracking shots as characters sprint through the winding, intestinal innards of the space station. The inherent wit of the exchanges between Keitel (who appears to be dubbed, a device that might have improved his performance in "The Duellists," too) and the robot he's driving crazy derive an added zing from expert cross-cutting.

Amis' scenario suggests an open and above board exploitation of devices that were manipulated for sinister innuendo in movies like "Demon Seed," "Alien" and "Dark Star." The sexual frustration and aggression are right on the surface. Keitel doesn't have hidden perverse motives, and as a result his crazed lust has an immediate humorous impact.

I'm not sure why the movie fizzles so drastically at the end. It appears that Donen expects audiences to derive more meaning from a view of the approaching Earth than the image itself can evoke. The story needs a witty capper, preferably similar in tone to the scene that provide "The Man Who Fell to Earth" with a startling, stirring exit.

It's a pity the movie can't be recalled and refitted with an effective ironic kicker. Maybe the present let-down could be put to advantage by conducting a belated writing competition and awarding a prize to the moviegoer who constructs the best last scene "Saturn 3" should have, given everything that's gone before. If the producers have their wits about them, they might even film the winning entry, substitute it for the current ending and salvage their investment by reissuing a new improved "Saturn 3."