For a three-act plot analysis, put on your screenwriter’s hat. Moviemakers know the formula well: at the end of Act One, the main character is drawn in completely to a conflict. During Act Two, she is farthest away from her goals. At the end of Act Three, the story is resolved.
Act I
The narrator and her friends arrive in the area of Northern Quebec where the narrator grew up. They are on the hunt for the narrator's father, who has apparently disappeared. Holed up in the remote cabin that had belonged to the narrator's father, the four friends hang out and work out their bizarre relationship dynamics (short version: there's, er, tension—sexual and otherwise). The narrator finds some drawings of her father's that make her think he's gone mad, which could explain his disappearance, but she soon comes to believe that these drawings are simply tracings of area rock paintings. She goes searching for the original paintings to confirm her theory.
Act II
While diving underwater near a cliff face to try to find one of these paintings, the narrator seems to have a kind of vision—well, something happens, but we're not quite sure what. Beneath her in the water, she believes she sees a fetus she aborted some time ago. This fetus is apparently the same child she claimed she had given birth to and left with her ex-husband. It turns out that she's been filling her own mind (and, by extension, the narrative) with some false memories, and this moment underwater causes the real story to you, know… surface. So, she is forced to start dealing with the sad truths in her past that were responsible for driving her away from her hometown in the first place.
After this epiphany, she decides she wants to stay on at the cabin and try to connect with the spirits of her deceased parents. (Oh yeah, and by the way: the narrator's neighbor came by to say some fishermen had found her dad's body.) To ensure that she can execute that plan alone—since her friends are being all kinds of dramatic and unpleasant to her—she hides and they leave without her. Before Joe leaves, though, she gets him to have sex with her, apparently so she can get pregnant.
Act III
The narrator takes a variety of steps and engages in various rituals seemingly designed to get her closer to the spiritual world (where she can run into her parents, of course). As part of that effort, she becomes almost like an animal, stripping off her clothes, destroying objects in the house, and sleeping outside in a lair she made for herself.
After achieving her objective by seeing both her parents, she prepares herself to return to the human world. And it's just in time, since Joe shows up while she's getting dressed to retrieve her, and is calling for her when the book ends.