Just when you thought the novelty of watching one actor play twins or clones (as in "Parent Trap," "Multiplicity" and "Adaptation") had worn off, along comes "Moon."

It stars a sensational Sam Rockwell in multiple roles, and he makes us believe we are watching identical but distinctively different astronauts standing side by side or face to face, conversing, playing Ping-Pong, literally going for the throat and trying to puzzle out just what is happening on the far side of the moon.

With no other actors in sight, Rockwell's task is more challenging than anything he faced in such films as "Confessions of a Dangerous Mind," "The Assassination of Jesse James by the Coward Robert Ford" or "Frost/Nixon."

In "Moon," he plays Sam Bell, an American astronaut who has spent the past three years living on the moon and mining Helium-3, now Earth's primary source of energy.

He doesn't reside in a crowded colony but at a base called Sarang where his only companion is a computer named Gerty (voice of Kevin Spacey) who asks questions such as, "Sam, is everything OK? ... You don't seem like yourself. It might help you to talk about it."

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'Moon'

3 stars = Good
Ratings explained

• Starring: Sam Rockwell, voice of Kevin Spacey.
• Rating: R for language.
• Web site: http://moon-trailer.blogspot.com

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Sam does talk, to himself or to his plants, conceding that three years is "way, way, way too long" to be alone, particularly since a broken satellite makes live communication impossible. He can send and receive taped messages from his wife and 3-year-old daughter but there's no semblance of real back-and-forth conversation.

Even his entertainment seems recycled, with old episodes of "Bewitched" and "The Mary Tyler Moore Show" for comic companionship. Two weeks before his expected return home, Sam gets into an accident in the lunar rover and nothing is as it was before -- or maybe everything is as it was before.

"Is there someone in the room with us?" he asks Gerty. "Who's the guy in the red robe and why does he look like me?" Is Sam moonstruck, or is this some cruel cosmic or corporate joke that Lunar Industries is playing on him?

Self-described "sci-fi geek" Duncan Jones, who happens to be the son of David Bowie, wrote the story with Nathan Parker. Taking place in the "near-future" when Helium-3 is sent back to Earth for fusion-powered generators, it's nevertheless a paean to the sci-fi films of the 1970s and early '80s.

Setting the story on the moon, as opposed to the outer reaches of space, makes it seem familiar (the colorless and cratered landscape) and not so far-fetched.

Just as Sam isn't sure if he's hallucinating, we're not sure initially what is going on. We learn as he does, and the pleasure comes not in explanation but in watching Rockwell manage to create separate but equal characters with a change of hair, pallor, expression and emotion.

Add to that the casting of Spacey as the voice of the Gerty 3000 Robot Assist. He can make Gerty sound soothing and motherly -- the robot helps Sam to dry his hair, for instance -- or menacing in a HAL 9000 way.

You don't have to be a sci-fi fan to say fly me to this "Moon." You just have to appreciate its moonstruck meditation on alienation, loneliness, homesickness and the man in the mirror.

Opens Friday at the Regent Square Theater and AMC-Loews.