As one might guess from the title, Another Earth opens with the announcement that a planet similar to ours has magically appeared in the sky in the form of a tiny blue star. Over the course of several years, the blue star turns into a full-blown mirror image of Earth that hangs in the sky even during the day. (The story isn't concerned whether or not this is scientifically possible; if that matters to you, watch something else.) In a somewhat distressing broadcast, a scientist on Earth makes contact with the other planet, and the person she reaches on the other end is...herself.

These developments are of interest to Rhoda (Marling), a young girl once destined for big things whose life is derailed by a personal tragedy. She is not the kind of person who gives up, but her forward momentum (a job as a high school janitor) is hesitant and tinged with regret, especially towards a man named John Burroughs (William Mapother), who was involved in the same incident, and whose life has also drifted off course as a result. He doesn't know her, but she knows him, and she is determined to apologize. She works up the nerve to go out to his house, but makes up a story about who she is and why she's there when they finally meet, and their continued interaction as a result of her story further complicates her guilt.

From beginning to end, this is Marling's show, and she delivers a subtle yet deeply moving performance that perfectly encapuslates a certain type of regret. Rhoda is not an overly sentimental person, and she keeps her emotions beneath the surface, but there is never a point in Another Earth where the viewer can't grasp every note of her complicated mixture of emotions whenever she talks to John, in scenes that blend heartbreak with triumph, and the brightness of possibility with the bleakness of truth. The story (again, co-written by Marling herself) combines this drama with the powerful "what if?" suggestion that hangs over Rhoda's head in the form of Earth II, without ever quite straying from the characters, although director/co-writer Mike Cahill accentuates the sci-fi material with talk radio broadcasts discussing the companion planet.

Going into the third act, there's a predictable but unbelievable plot twist, and the ambiguous final scene requires such vague things to happen off-camera that it's hard to tell what Marling and Cahill expect the audience to get out of it. There are also a couple of scenes, such as one where Rhoda stands in an icy field at dusk, which could've simply been excised for how fast the movie brushes past them. Still, the film isn't built around the other Earth or the twists and turns of the plot; this is Rhoda's emotional journey, and Cahill and Marling never lose focus.