How I think it all 'went down', + open discussion/interpreta tion

First off, we have to look at the clues and pieces laid out here. One has to remember that our narrator, like in ALL psycho thrillers with this brand of "twist" (think Fight Club, High Tension, etc.), is unreliable and provides a lot of room for potential plot holes, lapses in logic, and freedom of interpretation; however, some things do stand concrete here.

The film begins with Sarah out on the water front. Her dad pulls up to the house and she walks back, and complains she had a headache, to which her dad and uncle both think is an excuse for her to get out of working.

From there, it "starts", as far as we can tell, with the Polaroid camera— which I think was the first trigger that we witness as an audience for Sarah. If you watch closely, a look falls over her face when her dad and uncle start using the camera in the beginning to take pictures of the mold infestation, and it looks as if the sound/flashbulb cause her to remember something. So, that's the first seed.

Her uncle then leaves for town, and she moseys around the house downstairs for a bit and opens up a beer. The beer bottle may or may not be a "trigger", but I'm betting it is (I'll explain later). Right after she opens the beer, there's the knock at the door, and we meet Sophia. Sophia is not a real person, and, as everyone else has mentioned, is a figment of Sarah's imagination- maybe a coping mechanism, maybe an aspect of Sarah's personality; I'm not a psychologist, but she's definitely not a real person. Sophia mentions how she and Sarah used to "play dress up" all the time when they were kids, and that she has "some old photos lying around somewhere" and mentions that she'll look for them. All throughout this conversation, Sarah seems confused and embarrassed because she can't concretely remember ever knowing Sophia when they were children, but she goes along with it anyway since Sophia seems to remember her so well. Sophia asks if Sarah's in college, but Sarah says she and school "don't mix" and she's trying to make other plans, perhaps suggesting that she dropped out of college or was in some sort of trouble and is now trying to figure things out for herself. In the meantime, she says she's working for her dad. Sophia then mentions wanting to "do so many things" but being "unable to", and Sarah mentions how she has "holes" in her memory. Sophia says she'll be back later, and Sarah then insists that she does in fact remember her, probably so as not to hurt Sophia's feelings or make her feel weird by showing up like she did, but Sophia still seems totally sure of herself. "How could you forget?" she says, and then she takes off on her bike. This awkward encounter is all foreshadowing.

Later, while Sarah's cleaning out her room, she finds the red tin box which holds some of the photographs, but is unable to open it and she throws it in the trash bag. My guess though is that she had found other pictures elsewhere in the house already, because while she and her dad are investigating the first noise she hears upstairs (prior to her cleaning her room), he comes across some of them spread out on his bed and stuffs them into his suitcase; this indicates to me that Sarah already had found some of the photos and left them laid out for him to come across on purpose. We also see Sarah throw away a pink tutu that's lying on the bed while cleaning in her room. Both the red box and the tutu are other triggers, I think.

Then, notice how the attack that takes place on Sarah's dad occurs while Sarah is cleaning, almost immediately after she throws that red box and tutu in the trash. She hears a loud thud, finds her dad in the room full of the rugs and lamps, and a candelabra is lying on the floor, which is what she presumably clubbed him with. The extreme wound that we witness (where his eye looks practically gouged out) is an exaggeration of Sarah's psychosis. From this point on, everything we see as an audience is a complete free-for-all and is totally up for interpretation.

The chase through the house and property that goes on for a good 40+ minutes is just Sarah's adventure through her past and her subconscious manifesting itself and all of her repressed memories coming back to her. It's a hodge podge of Sarah running from both herself as well as reliving the terror of being abused/chased by her dad. I can't decide if the man in the camouflage(?) suit or coveralls or whatever he is wearing represents Sarah's father, the mentoring aspect of her personality, or perhaps both. It could represent her chasing after herself, trying to force herself to remember the truth, but it could also be fragments of how she remembers her dad when she was being abused. I believe that the makeshift bedroom we see in the basement with the child's bed is a place that she was probably forced to sleep (she may have been locked down there as a child).

Regardless, the billiard room scene is a culmination of all of this. At this point in the film, Sarah's uncle has returned home to find her frantic, claiming that intruders have attacked/taken her dad. They search the whole house and find nothing except the blood left on the floor from where Sarah attacked her dad with the candelabra; as we see later, he's tied up and covered with a plastic tarp downstairs.

So, Sarah goes around the house with her uncle and her dad is nowhere to be seen. Sarah then insists that they "check upstairs" once more, and her uncle agrees to it (I believe she snuck up there and turned the generator on, since her uncle says "that wasn't turned on before"). She goes with him up to the top floor where the billiard room is, and at some point attacks/confronts him and winds up getting a hold of the gun and shooting him in the stomach. It's unclear if she shoots him in the staircase or in the billiard room, but we do see blood smeared all over the walls when she comes downstairs a bit later, indicating that he may have been shot in the stairwell and struggled to make his way downstairs. Either that, or he was shot upstairs and dragged down there by her as we physically see. Remember, this is all up for grabs and could go either way, really.

In the midst of this (probably after the blackout scene, which is when she may or may not have shot her uncle), she recalls being molested on the pool table by her dad and having the photos taken. The scene in the bedroom after this with the blood on the bed, as well as the craziness of that bathroom scene is all just hallucinatory symbolism; the toilet on the wall with the blood is symbolic of vaginal bleeding, and the little girl in the bathtub full of beer bottles and bloody water suggests that she was probably violated/raped with the bottles by her drunken father. Pretty heavy stuff.

Then, wham! The electricity comes back on. Not sure how this happens, but she manages to find a way to get the lights in the house back on. This is where we see massive amounts of black mold covering the ceiling and walls of her bedroom where she had been cleaning. You could argue that the mold, which was not visible in the dark, may have contributed to her psychosis and/or been a psychoactive trigger to her memories (in reality, black mold exposure, especially that heavy, can in fact cause hallucinations). Remember how Sarah complained of a headache at the beginning of the film and had gone outside for the fresh air? It was already established that the house had a mold problem when her uncle discovers it growing behind the wall in the beginning of the film. This is a possibility, but I don't think it hinders the validity of her memories at all, but simply serves as another potential trigger.

So, she makes her way downstairs (this is when we see blood all over the walls of the stairwell I believe, leading down to the foyer) and Sophia is there. She's got the key to the box, which Sarah had all along. Sophia returned with those "dress up" photos, as well— they're in the trash bag on the floor. Remember, again, Sophia is a figment of Sarah's imagination. Think Fight Club. The red box is sitting on the table, and Sarah uncovers her father, who has been covered in the plastic sheet since she initially attacked him and beat him unconscious. Her uncle is lying on the floor on the verge of death, and Sarah's started a nice fire in the fireplace for her own little family intervention.

Anyway, back to the action here. Sarah, confused by Sophia's presence, attacks Sophia and slashes her hand with the garden shears, to which Sophia responds "Stop blaming yourself". Since Sophia IS in fact Sarah, the slash appears on her own hand, which suggests that Sarah self-mutilates as a method of dealing with the repressed memories (which is also probably why she has those inexplicable slashes across her wrist when her uncle picks her up after she breaks out of the cellar— think about it... where else did those come from?). It's here for the first time that we witness Sarah snap into her crazy self, which is what her uncle and dad both encountered when they were attacked by her. She's ranting and raving, pours beer all over her dad and suggestively rubs against him, asks if he wants to "play with her now", etc. Then, just by holding the beer bottle itself, she snaps back into her childhood and regresses like a little girl, crying and saying "ouch, daddy, it hurts" (more implication that she was violated with bottles), and then snaps again and tells her dad "Shhh, we wouldn't want mommy to hear" (something her dad often would have said to her while this sexual abuse was going on). This whole scene reminds me of Mrs. Voorhees in the original Friday the 13th, really.

Then, for a second, we're led to believe that maybe, just maybe, Sarah is absolutely nuts and made all of this up in her head and her dad may be totally innocent, but the second she lets him free, his violent and domineering nature comes right out and he starts whipping her with his belt as she lies on the floor crying and screaming at him like a small child (keep in mind, she's probably in her early 20s and she's acting like she's 9). This just tells me that this is in fact what he's actually like, and this is probably the kind of treatment she got from him as a kid. The uncle, who seems to have been more of an accessory to Sarah's childhood abuse, feels remorse and tries to stop him, and dad dismisses him and says something about how he "liked to watch" too. This small confrontation between the brothers gives Sarah the 5 seconds she needs to grab a hold of the sledgehammer and thwack daddy upside the head once and for all. She decides to have mercy on her uncle since he appears to have been remorseful and had a more passive part of the abuse. Fact of the matter is, she already had shot him and he was probably going to bleed to death anyway. Sarah walks out of the house, mission accomplished.

The end.


So, since this interpretation is extremely tl;dr, here's a shortened crux without the nit-picky details:

1) Sarah was abused by her dad and uncle in that house in a variety of ways when she was a little girl— violated with beer bottles, raped, locked up, beaten, forced to pose for pornographic photos, etc, etc.

2) She's very seriously (and understandably) psychologically f##### up by all of this.

3) Being back in that environment forces her to recall and relive her abuse for a variety of reasons.

4) She finds actual photographic evidence of the sex abuse when she stumbles across the Polaroids, which causes her to have a mental breakdown, so she attacks dad while her uncle is gone, and then eventually attacks uncle as well when he returns from town.

5) Family intervention in the living room ends in a well-deserved blood bath and justice is served.