Nell (Ellen Page) and Eva (Evan Rachel Wood) live with their widowed father Robert (Callum Keith Rennie) in the forest of the Pacific Northwest in the near future. The power fails and never comes back on, as does all communication and water service.
They go into town for supplies and meet Stan (Michael Eklund), who tells them that there are no gasoline or supplies available anywhere. They return to their home in the woods and stay there, saving their gasoline to use the car or generator in an emergency. Robert has an accident with his chainsaw while cutting wood and dies. The two survive by learning to find food in the woods (somehow it's always summer there.)
Nell's boyfriend Eli (Max Minghella) comes to the house and asks Nell to walk to Boston with him because he's heard rumours that things have "started up again" there. Eva refuses to go with them. Nell leaves with Eli, but returns to Eva the next day, feeling unable to leave her sister.
While Nell is away picking blueberries, Stan shows up and rapes Eva. She is deeply traumatized and refuses to leave the house. When she realizes she's pregnant Nell wants to terminate the pregnancy but Eva refuses, saying "I don't want to lose anything else." She goes into labour just as the roof of the house, which was undergoing repair at the start of the film, begins to cave in from accumulated water damage. They go into the forest where Nell delivers the baby.
They return to find the house is moldy. Eva declares they should burn down the house with the remainder of the gasoline, arguing that they don't need electricity to survive, and they'll be safer in the woods. Anyone who comes looking for them will think they died in the fire. Nell relents and they gather supplies, burn down the house and leave to live in the forest.