How It All Goes Down
The narrative comes to us via an unnamed narrator, who happens to be bringing three friends to the area of Northern Quebec where she grew up. The group includes her boyfriend Joe and another couple, David and Anna. The story begins with the group's trek through the more populated areas in the narrator's old stomping grounds (e.g., an unnamed "city" and a place she refers to as "the company town") to reach the remote cabin where her father had been living—that is, before he disappeared (dunDUNDUN).
We soon learn that the narrator had received a letter from a family friend, Paul, informing her of her father's disappearance—hence the homecoming after what appears to have been a long absence. The narrator had hoped her father would have turned up by the time she got there (so she could just scoot right back home), but no such luck.
When she and the others arrive at the cabin, she finds evidence that her father hasn't been there for a while. They look for him a little bit, but the narrator quickly comes to believe that the search is futile—they'd need a lot more people-power to search the entire island for him.
So, they occupy themselves with fishing, swimming, shooting film for a future amateur cinematic project, and reading—oh, and some pretty intense psychological warfare-romantic drama, too. For example, Joe decides that this recon mission to find the narrator's (probably) deceased father is the perfect opportunity to declare his love and propose. (Hey, why not?) The narrator apparently has some issues with the "L word," so she is less than receptive—which does not go over well with Joe.
Meanwhile, David and Anna seem to have a lot of tension between them. David is, shall we say, not the nicest guy; his main joy in life seems to be making fun of Anna and even humiliating her in front of others. To make matters worse (read: skeezier), he openly hits on the narrator in front of Anna and Joe, and at one point he even propositions her for sex (he justifies the invitation by telling the narrator that Joe is off having sex with Anna). Ew.
So, yeah, if they had had Facebook back then, "It's Complicated" would have been the relationship status for this wacky quartet, and the faults and fractures in these relationships increasingly take center stage as the novel progresses.
Alongside all of this, the narrator is digging deep into her memories, thinking about her family, her childhood, and a husband-child that she had left behind at some unspecified moment in the past. Despite the fact that she gives up searching for her dad almost immediately and assumes he is dead, she still wants to know what could have happened to him. When she finds some drawings her father made of weird-looking, seemingly mythical beasts, she briefly considers the possibility that he might just have gone crazy (and might still be alive). However, she soon realizes that the drawings were tracings of nearby rock paintings, not figments of a demented mind, so she goes back to thinking he's probably dead.
She decides to go in search of the rock paintings, trying to gain confirmation for this little bit of the "real" story she's pieced together from what her father left behind. She doesn't have much luck when she takes the others out looking for the first one, but her second attempt—which involves going on a solo dive to look at the side of a nearby cliff—is a bit more… interesting, shall we say. While she's down there, she sees something (she believes it to be the fetus of a baby she had aborted years before) that inspires certain memories or realities to "surface" in her own brain, and we learn that not everything she's told us is entirely accurate.
For example, speaking of the fetus, we learn that the child she supposedly abandoned with her ex-husband was actually never born; she had an abortion (and the father of the child wasn't her husband, but a married man with whom she was carrying on an affair). So, that's some news. After this incident, the narrator comes to believe her father wasn't just cataloguing these existing rock paintings, but also compiling a list of places where these kind of spiritual events or "oracles" (17.26) could occur.
In the wake of this underwater epiphany, she's suddenly not in such a hurry to leave. So, the day she and the others are slated to head home, she opts to stay behind. What happens from there is definitely up for interpretation, but she appears to pursue some kind of spiritual communion with her deceased parents (oh yeah, and we should mention that her father's body was found the day before they were supposed to go home).
To get that ball rolling, she engages in a lot of ritualistic behavior, looking for "signs" for what she should be doing at any given moment to gain access to her parents. In her view, this process means getting a lot cozier with nature and her more animalistic side. She destroys basically all of the belongings inside the cabin, strips her clothes off, and makes herself a lair outside. Um… yeah, grief is powerful, y'all.
Eventually, she appears to get what she wants and has visions of both her parents. After that occurs, she seems ready to kind of return to the world. She comes to the realization that she needs to stop being a victim and understands that she is not powerless (a belief she had used to convince herself that she couldn't hurt other people). With that epiphany, she goes inside and gets dressed in her clothes (even though she had slashed them all up).
A boat arrives with Paul and Joe, and Joe calls for her. She seems to realize she loves him, and she thinks about what going back with him would mean as she watches him. We are left on a cliffhanger, as we don't know if she ever answers his call.