Rhoda Williams (Brit Marling), a brilliant 17-year-old girl who has spent her young life fascinated by astronomy, is delighted to learn that she has been accepted into MIT. She celebrates, drinking with friends, and in a reckless moment, drives home intoxicated. Listening to a story on the radio about a recently discovered Earth-like planet, she gazes out her car window at the stars and inadvertently hits a stopped car at an intersection, putting John Burroughs (William Mapother) in a coma and killing his pregnant wife and young son. After serving her four-year prison sentence, Rhoda chooses to work with her hands and to have minimal contact with other people, becoming a janitor at her former high school.
Hearing more news stories about the mirror Earth, Rhoda enters an essay contest sponsored by a millionaire entrepreneur who is offering a civilian space flight to the mirror Earth.One day Rhoda sees John laying a toy at the accident site. She visits his house, intending to apologize. 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 a cleaning service. John, who has dropped out of his Yale music faculty position, has been letting his home and himself go, and accepts Rhoda’s offer. He has no idea who she is, and when she finishes asks her to come back the next week. In time, a caring relationship develops and they have sex.
Rhoda wins the essay contest and is chosen to be one of the first to travel to the other Earth. John asks her not to go, believing they might have a future together. She finally decides to tell him the truth about who she is. He is upset and throws her out of the house.
Rhoda hears an astrophysicist talking on television, describing a “broken mirror” hypothesis which states that upon the sighting of the twin-Earth the synchronicity of events happening in both the earths was broken. Rhoda rushes back to John’s house, but he refuses to let her in. She breaks into his house, and he begins to strangle her. He stops, and when she recovers she tells him about the theory and that there might be a possibility for his family to still be alive on the other Earth. She leaves him the ticket. In time, she learns that John accepted the gift and becomes one of the first civilian space travelers to the other Earth.
Four months later, on a foggy day, Rhoda approaches her house, discovering her other self from Earth 2 standing in front of her.