ANYONE WILLING to suspend disbelief and ignore awkwardness for 90 minutes will find much of Marshall Brickman's new comedy about an extraterrestrial from the constellation Orion comical. Simon has many of the trappings of a Woody Allen movie, particularly reminiscent of Sleeper in its science-fiction silliness, and it's no wonder--Brickman collaborated on Sleeper, as well as Annie Hall and Manhattan. Both moviemakers tend toward the intellectual, so it comes as no surprise that Brickman's characters cite Wittgenstein. But Allen's products are by far the more polished of the two; Brickman often has wonderful ideas that just don't quite come through on screen.

His idea in Simon: five brilliant but loony scientists at the "Institute for Advanced Concepts" decide that the American public is ready to meet a full-fledged alien in its already alienated society. They search for a middle-aged male orphan whose memory they plan to deprogram, and then convince him that his mother was a spaceship who dropped him on earth from an advanced civilization where humans are made the way we make toasters. One Simon Mendelssohn becomes their victim, an untenured professor of psychology who is slightly off the deep end already.

The scientists finally persuade Simon, played flawlessly by the unflappable Alan Arkin, that he's an extraterrestrial who has come to save the world. But they turn him into a monster, whose ego expands exponentially, and who eventually acquires his own television station--Simon, the fourth network--commanding people to terminate their fondness for disco, Muzak in elevators, and other socially irritating habits.

Brickman's story is a complicated one to retell, but not to unravel. Simon eventually unwinds from his identity crisis to perceive the truth about himself, and escapes with his girlfriend from the Institute after the Pentagon has come with orders to kill him. Along the way, he meets a tribe who worship the Sacred Box, and hold services in which the reader preaches from TV Guide--a comic touch that succeeds in theory, but not in practice, and reminds us that this is, after all, Brickman's first movie on his own.

Nonetheless, flashes of brilliance coupled with wonderful individual performances set Simon apart from run-of-the-mill, hack comedies. Arkin is totally insane in this movie, and well he should be. His unabashed portrayal of this bizarre "visitor from the stars" captures plenty of subtlety. His funniest scene in the movie--when he extricates himself from a sensory deprivation tank he's been kept in for almost 200 years--is so good you want to rerun it many times so it'll settle in your memory. His brain loses 500 million years of evolution in the process, and in the following minutes in the movie, Arkin acts out all of human evolutionary history--from the very first algae, through the many amphibian and primate stages, with a stop at Ramapithecus along the way, faster and faster until he discovers sexuality, and then language. The narrator to this Biology-book scenario notes a sudden pause in Simon's euphoria--probably about the time of the Original Sin--and concludes that "... there's too much joy. He gets guilt and religion."

BRICKMAN'S ACERBIC but amusing humor fills Simon, which he's directed as well as written. Some of his jokes are priceless--one of his scientists declares that "the Nixon who came back from China in 1972 was not the same one who went." Just as the successes of the screenplay must be attributed to him, so must the almost amateur direction, in which characters seem at a loss as to what they should do and how they should react to one another. An early scene comes to mind in which Simon and his girlfriend mutter indistinguishable bad jokes and move awkwardly in Simon's office. The picture also lingers at the outset: Brickman should introduce us to Simon and his life first, and then the scientists' mad concoctions, not the other way round.

Brickman is fortunate to have the talented Arkin and his cohorts in this movie. Austin Pendleton as the playful, Machiavellian Becker, and Fred Gwynne '51 as an oafish Pentagon general are particularly effective in their roles, and Madeleine Kahn, though not blessed with a very demanding part, provides some of the funnier moments at the Center. They are all able to keep their jokes alive, and Arkin's evolution from minuscule plankton to neurotic 20th-century man should be recorded as one of the funnier scenes in contemporary movie-making. But one scene doesn't make a whole movie. We can't expect Brickman to be brilliant for the entire 90 minutes, but there are times he falls so far short we can't help feeling we're watching an undergraduate movie in a film course.

Woody Allen probably enjoyed this movie, and well he should have--after all, imitation is the highest form of flattery. But Allen is accomplished, and succeeds at wearing the multifarious hats of director and writer much better than Brickman. His collaborator has the jokes, the ideas and the talents to be a major movie-maker. But right now, Brickman is a stand-up comedian with an audience that laughs at every third joke.