PARK CITY, Utah  – The best science fiction tells stories about people in extraordinary environments or situations that serve to open up the vast, still largely unexplored terrain of the human heart. Mike Cahill's "Another Earth" is science fiction at its best.

The movie is a bolt from the blue here at Sundance, where it is screening in the U.S. dramatic competition. Its director of programing, John Cooper, admits the festival wasn't even tracking the title and the film arrived here without much heat until its Monday afternoon debut. That's all changed now.

Perhaps as startling as the film itself is the background of its creators. Cahill's background is documentaries, largely with National Geographic, while his co-conspirator, Brit Marling, worked as an investment-banking analyst before taking a sabbatical to co-direct the Havana-based doc "Boxers and Ballerinas." The two wrote the screenplay for "Another Earth" with Cahill directing, producing, shooting and editing while Marling acts as his co-producer and star.

The simple story has overtones of the recent "Rabbit Hole." Marling's Rhoda Williams is a brainy young New Englander, recently accepted into MIT's astrophysics program. Distracted while driving one night, she causes a terrible accident that kills the entire family of a celebrated music composer, John Burroughs (William Mapother), leaving the man in a coma.

The distraction is crucial here. She was gazing out her window at a blue object in space. It seems astronomers had just discovered another planet hidden until then behind the sun. During her four years in prison, further scientific inquiry reveals this planet is a duplicate Earth. In other words, man is confronted by the existence of a parallel reality. A scientist attempting to contact Earth 2, as it is dubbed, is shocked to find she is talking to her other self.

Upon release, Rhoda is motivated to go to John's home to apologize or ask forgiveness -- she isn't really sure what she wants to do -- but chickens out at the last moment. Instead she introduces herself as an employee of a cleaning service offering a free trial.

The composer, gone to seed and the bottle following the loss of his family, could use some house cleaning. So Rhoda winds up going weekly to the lovely old white house without telling her employer of their tragic connection.

A relationship develops when the composer discovers the inquiring, restless mind of the young woman. Slowly he awakens from a second, self-induced coma to rejoin the human race.

All the while, Earth 2 looms larger and larger in the sky. The metaphysical question its presence poses is what if these people could confront themselves in a parallel continuum? Could things possibly be different and the accident didn't happen? Or at least can one learn from one's other self?

Commentators on background TVs and radios noodle around with these notions, as pundits reliability do. This allows the personal drama with all its hidden emotions and one terrible secret to play out in a realistic way while these questions loom over the drama.

Cahill shoots in cool tones as if these two people exist in eternal night. The world of the composer's home, once chaotic but gradually turned organized and livable by his new maid, reflects his rebirth. He is back among the living.

Everything comes to a head when Rhoda enters a contest to be among the passengers on the first space ship that will fly to Earth 2, an opportunity for her to reach out to this alternate reality.

The movie is essentially a two-hander featuring Mapother and Marling. Mapother, Tom Cruise's cousin, is a grave, slightly surly presence initially but he projects warmth buried beneath the self-pity and grief. His John Burroughs wants rescuing. Marling, a beautiful and slender blonde, has a face of utter transparency. You read her every swing between hesitancy, determination and back again.

So too does the movie swing between hope and despair. One aging character, a fellow custodian at the high school where Rhoda finds janitorial work, gives into despair. Rhoda does too at one point but when she recovers, Earth 2 shines brightly down on her with its potential for redemption.

The outstanding credits across the board demonstrate once more how much can be achieved in film even on a modest budget. Meanwhile, Sundance can boast of another discovery and the birth of two more stars in its already crowded firmament.