Little movie. Big jolt. “Moon” is the early summer movie no one saw coming – a small movie with a decidedly modest budget that shakes you into big remembrance of a simple fact about movies: some of the best and smartest we’ve ever had have always been sci-fi.

For decades now, we’ve gotten used to the genre commandeered by the mega-geekdom of technomytho- windbag George Lucas and all of his onscreen progeny (who include all the silly thundering Legos in “Transformers”).

Once upon a time, sci-fi was for grownups and smarties and people who cared about good movies, not little ones and pugnacious fanboys cherishing their right to remain Beasties as long as society will let them.

That’s the kind of movie “Moon” hearkens back to.

“Moon”

3 1/2 stars (Out of four)

Rated R

Sam Rockwell and the voice of Kevin Spacey in a sci-fi tale of a future energy miner on the moon’s surface who starts seeing both hallucinations and previously ignored truths in his loneliness. Opens today in Dipson Eastern Hills.

The filmmaker is Duncan Jones, and he’s David Bowie’s son, a fact that no doubt bedevils a very talented young filmmaker who needs no pedigree and deserves all the credit in the world for individuality, even defiance, in his era. It’s just that you’d have to have the acuity of a tomato not to wonder how this tough little post-”2001” fantasy was influenced by growing up the son of the man who not only gave us “Space Oddity” but starred in that terrific Nicolas Roeg sci-fi minor classic “The Man Who Fell to Earth.”

“Moon” — written by Nathan Parker from Jones’ story— starts in territory that can’t help but remind everyone of Kubrick’s “2001: A Space Odyssey.”

In some distant but very familiar future, humankind has taken to mining the surface of the moon for a helium isotope to turn into energy. The movie we see is about Sam Bell (played by Sam Rockwell of “Confessions of a Dangerous Mind”). He’s at the end of a three year stint as a one-man workforce and loneliness is beginning to drive him bats. He’s seeing things — sudden apparitions of beautiful women etc.

His only companion is Gerty the computer, voiced by Kevin Spacey.

Gerty is like a warmer version of Hal in Kubrick’s “2001.” In other words, Gerty’s almost as duplicitous as Hal but doesn’t seem given to paranoia and murderous megalomania the way Hal was. Gerty does everything for Sam from supplying computer passwords he forgets to heating up his morning beans. With Kevin Spacey’s voice, he’s always courteous and never snippy, like some computers we can think of.

At times, Gerty seems genuinely helpful, against his corporately programmed interests.

Or is he?

And that’s only part of the movie.

The other part is what happens when Sam suddenly comes face to face with another Sam in his space station: an identical, flesh and blood Sam in better physical shape who, in fact, saves his sorry, ragged behind from a moon surface crash that should, by all rights, have left him for dead.

It turns into a doppelganger movie about extreme blatant emotions and hidden ideas. It’s genuinely suspenseful — extremely so, in fact. And as derivative as it is, it still manages to be completely fresh because it’s derivative of the right things, not the wrong ones.

In other words, it builds emotionally on “2001.” And if the space world it shows you is definitely a variant of George Lucas’ early “THX-1138,” which had the daring to show us outer space hardware that was used, worn and dirty, rather than gleaming white and pristine, it always does so cleverly. This is a space station manned by a solitary guy with little if any interest in housekeeping. That’s Gerty’s deal, not his. He’s only as orderly as he has to be.

Sam lives for bad TV reruns and infrequent messages from his estranged wife and newborn daughter on earth. He’s literally counting down the hours before his return, after his three year contract is up.

I wish the ending of the movie were tougher, in truth. I think the movie loses interest in human malevolence—especially of the corporate kind— at the wrong time.

And I think this completely unexpected movie house animal should have had a sting in its tail — an ending to leave you with a little venomous twist in your head as you left the theater.

It’s still a good one, though. And you don’t have to be a fan of Duncan Jones’ father’s to be on big lookout for the next thing Jones does.

The movie isn’t big but his talent is.