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Remember Gareth Edwards' Monsters from a few years back? It was set in a world where giant alien creatures had come to Earth and most of Mexico was now sealed off to keep the monsters in quarantine - but the film didn't care about them and was more concerned with the mumbly mismatched relationship between two not particularly interesting humans. The Monsters of the title were so far in the background they need not even have been there: they were practically redundant in their own film and could have been replaced with an Earthbound menace like drugs gangs or political extremists. That's not to say that Monsters is a bad film, merely that it's a noodly character drama and not a monster movie as claimed in the title and poster and promotional material.
In a similar way, Another Earth is only very slightly about the momentous discovery of a mirror Earth in the sky, and spends most of its time with Brit Marling's young ex-con as she tries to expiate her guilt over the car crash that killed a young family by working (unpaid) at the house of the sole survivor (William Mapother) who's been in a coma and so doesn't recognise her as the drunk driver who caused the crash four years ago. She cleans his house, brings a little joy into his life, starts a physical relationship with him....but she's also applied to be one of the select few to voyage to Earth 2. What if she wins? What happens when Mapother finds out who she really is?
There's a bit of stuff about how Earth 2 was absolutely identical to Earth 1 until the moment we saw each other, then the synchronicity was revoked and maybe that fatal accident didn't happen on Earth 2, so maybe "they" are still alive up there. (Presumably no-one on either Earth 1 or Earth 2 wonders if their equivalent was killed in a car smash four years previously.) But much of the film is a dull drama of guilt and penance, with Marling desperately seeking forgiveness and Mapother unaware there's anything to forgive. That's not necessarily a bad area for a drama, but it doesn't really need the Star Trek/Doctor Who/Red Dwarf mirror Earth stuff hanging up there.
Maybe this is a case of reviewing the film it isn't rather than the film it isn't, but frankly, just as with Monsters, I was much more interested in it as SF than as a low-budget indie and for me it's the SF angle that's left on the sidelines. The other obvious point of comparison is Lars Von Trier's gloomy Melancholia, which at least has the gorgeous slo-mo opening and properly apocalyptic ending which Another Earth's tiny budget and harsh handheld digital look can't hope to match. That perhaps helps the naturalistic, noodly indie feel, but for those who might think the sudden discovery of a new planet should take priority over a joyless character drama, it leaves Another Earth seriously lacking and not much fun at all.