Ann's living alone in the woods after your standard zombie plague apocalypse. She sleeps in her car, forages for berries, tries catching game, and occasionally ventures into some houses vaguely nearby to look for food. One day she finds Olivia and her stepdad Chris staggering down the highway. She helps them out, with reservations. They're heading north, in the direction of a radio broadcast. It's in French, they don't know what it's saying, but they figure it's worth taking a chance on it. Ann, however, is content to stay right where she is. Well, more that she's unwilling to leave this particular place.
There's some gradual thawing, the trio growing more comfortable around each other, which leads to its own problems at the climax of the film, but Ann's still refusing to let go of the memories that hold here there.
I spent the whole movie expecting that trusting other people was going to end badly for Ann. They were going to steal all her stuff, try to kill her. Which says more about me than the movie, really. It really seems to be about the pointlessness of playing the martyr. Chris mentions at one point that yes, sometimes he feels sad about the loss of Olivia's mother, but other times he chooses not to think about it. You can be sad about something, about the loss of someone, without having to be sad and suffering over all the time.
Ann is making the conscious effort to try and stay in mourning, rather than move forward. She's going to stay right there, wearing the figurative sackcloth and ashes until the zombies lurking around those houses run out of corpses to eat and venture into the wilderness. At which point she will probably die. It doesn't have to be that way, but that's the decision she'd been making. Now she's given an offer to go towards the unknown with two other people, or keep going as she is.
It's a quiet movie, punctuated by brief moments where things get tense. There are a couple of lighter moments, not funny so much as the characters not being focused on survival for a few minutes.