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Duplicates are a staple of sci-fi, and filmmakers continue to seize on the idea. Among recent efforts one need only think of “Moon,” with its doubling of the protagonist. But “Another Earth” trumps that by positing the discovery of a whole “mirror” planet where everything on our own, it seems, is replicated.

But that premise is merely the backdrop for a much more conventional story of guilt and redemption. Rhoda (Brit Marling), an ambitious girl who celebrates overmuch after getting accepted to MIT, is responsible for an auto accident in which a pregnant woman and her young son are killed. Sent to prison for intoxication manslaughter, she emerges after serving her sentence morose and withdrawn. Taking a job as a janitor in her old high school, she visits the sole crash survivor—John Burroughs (William Mapother), a Yale University music professor who’s become a recluse still in deep mourning over the loss of his wife and child. She poses as a maid offering to clean up his mess of a house, and he quickly hires her on a regular basis.

The rest of the film basically follows the halting growth of friendship between the two, a relationship constantly threatened, of course, by Rhoda’s failure to identify herself. There are a few touching moments in this process, but overall Marling and her co-writer (and director) Mike Cahill offer it up in a plain, scruffy style marked by jumpy handheld camerawork and some pretty ludicrous narrative turns. (At one point, for example, the elderly janitor Rhoda works with encourages her with some pretentious bromides, and at another Burroughs takes her to a campus auditorium for a private recital—which he gives on a singing saw, a typical instrument at Yale!)

This personal story is integrated into the overarching double-earth idea by Rhoda’s entrance into a contest to join the first crew that will fly off to the duplicate planet. The outcome of her effort is meant to bring closure to her relationship with Burroughs and to the picture as a whole. While the precise nature of the ending won’t be revealed here, it must be said that if one thinks through the implications of what happens, given that the other earth is supposed—I think—to have experienced much the same events that have happened on this one, it makes little sense. (Even if that’s not the case, the ending’s implications are at best doubtful.) Of course, one might say the same about much science-fiction writing.

Under the circumstances, Mapother and Marling do reasonably good work, though Cahill’s ostentatiously grubby direction and editing don’t help them much. Nor is the picture attractive simply to look at—Cahill’s hand-held camerawork is jumpy—though it must be admitted that achieves a mood that reflects the characters’ downbeat emotional lives.

“Another Earth” ends up seeming like the sort of picture that can be tolerated at a festival like Sundance, where it was warmly received, but is totally out of place when it escapes such a specialized venue.