Rhoda Williams (Brit Marling) is a high school student and amateur astronomer, who has recently been accepted to MIT. She celebrates with friends and drives home intoxicated. Listening to a story about a planet that has just appeared at the sky on her car radio, she looks out her car window up to the stars, and inadvertently slams her car through a stopped car at an intersection, putting John Burroughs (William Mapother) in a coma, and killing his pregnant wife and son. Rhoda receives a 4-year prison sentence, but since she is a minor, her identity is not communicated in the papers.After serving her prison sentence, Rhoda is picked up by her parents to live at their home. Radio broadcasts provide more information about the other planet, which has come much closer over the last 4 years, and is now clearly visible in the sky during the day. It is a complete copy of Earth, with continents, cities, oceans and all. Scientists are attempting to make contact, but they have difficulties broadcasting through all the signals coming from 'Earth 2', as they call it. Rhoda's brother tells of an essay contest sponsored by millionaire entrepreneur Keith Harding, who is funding a space flight to Earth 2, and grants a free ticket to the civilian who has the best motivation to join. Rhoda chooses not to use her old room, but takes the austere attic instead. As an ex-convict, she knows that she will no longer be accepted at MIT, so she accepts a job as a janitor at a local school, wanting to physically clean things with her hands but not do too much thinking. She builds something of a report with an old blind janitor called Purdeep (Kumar Pallana) but avoids contact with others as much as possible.After cleaning the school for a while, she returns home one day after work, and goes to the intersection where she caused the fatal accident. She sees John Burroughs pulling over, placing one of his son's toys on the curb in memory of his deceased family. She follows him home, but not knowing what to do, she remains outside, partly undressing and exposing herself to the cold while watching Earth 2. She is found before she freezes to death, and treated for mild frostbite.While recovering, Rhoda enters the essay contest, explaining how as an ex-convict, she is excluded from most privileges. However, in the old days of naval exploration, it would often be ex-convicts who dared to take the risk of venturing into the unknown, and were among the first to set foot on new worlds, so this would make her a likely candidate to be among the first to visit Earth 2.Rhoda does an internet search on John Burroughs, discovering that he used to be a celebrated composer and musician. She also reads that John awoke from his coma when his entire family was already buried. Riddled with guilt, she again visits John's house, hoping to apologize for the harm she did to him. He answers the door and she loses her nerve. Instead, she pretends to be a maid, offering a free day of cleaning as a marketing tool for Maid'n Haven (a New Haven based maid service). John, who has nearly dropped out of his Yale music faculty position and is now living in a depressed and dirty stupor, agrees to Rhoda's offer. She starts cleaning, and John notices her interest in a telescope lying in one of the rooms. When she finishes, John, who still does not know she is the person who killed his wife and son, asks her to come back next week. Rhoda tells him someone will come, but it may not be her.Back at home, Rhoda's brother shares news that he has been accepted into UConn. Rhoda practices a confession speech for the next time that she meets John Burrows. She returns to his house, but he immediately puts her to work, not giving her the opportunity to say much. After her cleaning is done, he gives her a check and shows her out, again totally oblivious of who she is.A television broadcast shows a successful attempt by scientists to send a signal over to Earth 2. One of them finally makes contact with a person on the other side. Not only are there humans who speak English there; one scientist is startled to discover that the woman she is speaking to is her exact copy in every aspect: same name, date and location of birth, even the occupation and childhood memories match. Rhoda's family, however, thinks it is just an elaborate hoax.The broadcast immediately sparks a scientific debate. Physicists theorize that Earth 2 is a mirror planet, an exact copy of our Earth, including all of its inhabitants. Philosophers open up the possibility that one could meet and talk with their exact copy, a person who may be identical but has led a different live.Rhoda cleans John's house again, and develops a caring relationship with him. They like each other and are intelligent and compatible conversationally. Rhoda genuinely wants to be of service to him as he gradually comes out of his depressive stupor and regains some joie de vivre. They play a video game and John allows Rhoda took look at Earth 2 through his telescope. Afterwards, he offers her a ride home, during which they discuss their potential copies on Earth 2. When asked what she would say to her copy, Rhoda responds "Better luck next time" before she says that she would probably be speechless. While dropping her off, John realizes that he doesn't know her name, so she tells him it is Rhoda. Again, no suspicion in John.At work, Purdeep seems to notice how conflicted Rhoda is. He tells her to keep her mind clear to obtain peace of mind. At John's house, they arrive on the subject of the first expedition that plans to go to Earth 2. Rhoda tells him a story of a cosmonaut who is alone in space, plagued by a ticking sound that he cannot stop. In order to maintain his sanity, the cosmonaut decides to listen to the sound as if it were music, obtaining peace of mind. She tells him about the essay, but John is skeptical about the expedition. He uses Plato's allegory of the cave as an illustration of how mankind may not be ready to accept the realization that they are no longer unique, since they still believe that they are the center of the universe. When John learns that Rhoda has washed a piece of clothing, presumably from his deceased wife, he becomes enraged and tells her to leave.A radio debate also warns about the potential dangers of Earth 2, since it also has humans with the same access to weapons and a comparably suspicious nature, which are ideal ingredients for conflict and mutually assured destruction. At work, Rhoda asks about Purdeep, who is in the hospital. He has poured bleach into his ears (which he had already done to his eyes once) as he was reportedly "tired of seeing himself everywhere". Outside, she meets John who says he is looking for her, but nobody at Maid'n Haven has heard from her, and all his checks have gone uncashed. She gives some quick explanations, and accepts his apologies for his earlier outburst. He invites her to his university, where he gives her a private concert of a new piece of music that he has been composing. Afterwards, they go home, where they finally give in to their feelings, and have sex. John tells about how he was finally content with his life, when a drunk driver killed his wife, son and unborn daughter. When he woke up from his coma, he was so angry at the perpetrator that he let his brother attend the lawsuits, and never learned that person's identity since it was a minor. Again, Rhoda doesn't dare to confess.Rhoda visits the hospital to see Purdeep who is now deaf and blind, hoping to hear an explanation for what he did. She hugs him and he immediately guesses the reason why she is there, and tells her she knows why he did it. She writes the word FORGIVE in the palm of his hand, making the old man weep.Rhoda comes home and hears that the people from the space flight have called. She calls back and is answered by Keith Harding, who says that he was moved by her motivation. He sympathizes with her, hinting that he once came close to becoming a convict himself, so the seat on the flight to the other Earth is hers. She goes to John to tell him that she has won the space flight, and that she will prepare for a training program in the Mojave Desert. John seems enthusiastic at first, but then asks what testament and funeral arrangements he has to make in case she does not survive the flight. Finally, he flat out asks her not to go, since they are so close to something now. She says that she will tell him a story, and if he still wants her to stay afterwards, she'll stay. She tells him about how naive she used to be, and in her recklessness, she killed his wife and son. When she went to him to apologize, she lied in the hope that she could make his life somewhat better. But now, she fears that she is doing this to make herself come to terms with what she has done. Devastated by this confession, John angrily forces her out of his house.Rhoda returns home, finding out that the media have discovered that she is the winner of the space flight essay, and that she is an ex-convict. She is flooded with questions from reporters around her house, and flees inside. Her parents are upset that she didn't mention this to them, and want to know what she plans to do. In a telecast, they hear a theory that the citizens of the mirror Earth were identical to those on our Earth in every way, until the moment that they learned of the other's existence. From that point on, the identical people on the different Earths probably began to deviate in small ways, changing their actions and perhaps the course of their lives.Rhoda returns to John, but he refuses to open the door. She crawls inside through a window, and tells John about the theory of broken synchronicity which may have occurred four years ago. She gives him the ticket to the other Earth, tells him that there is a chance that his wife and son might still be alive on Earth 2, and leaves.Four months later, Rhoda has switched the attic for her old room, and watches a news program about the space flight to Earth 2 that is scheduled in a few days. She sees John giving an interview; he has undergone the space flight training in the Mojave Desert, and says that he feels privileged and proud to be among the first to go. Rhoda is happy, and after work, she leaves to go home. When approaching her back door, she is startled by a confusing sight: her identical copy from the other Earth standing in front of her. She is wearing fashionable attire that contrasts strongly with her own mundane clothes, suggesting that her twin has had a much better life for the last 4 years.