Martha (Elizabeth Olsen) has broken away from the communal farm where she's been living -- the movie never uses the word "cult," but it would certainly be appropriate -- and is staying with her sister Lucy (Sarah Paulson) and brother-in-law Ted (Hugh Dancy). She's been on that farm for several years, and is having trouble readjusting to life away from its charismatic leader, Patrick (John Hawkes). Her behavior's not always appropriate, and she's clearly been deeply affected by whatever happened to her during her years away.
Durkin's movie jumps back and forth between the farm and Lucy's lakeside summer home, gradually filling in the pieces of the puzzle. At the same time, it's painting Martha as so emotionally and psychologically unstable that we're not entirely sure how to take those farm scenes; if these are supposed to be Martha's memories of what happened, how much can we trust them, given her obvious psychological damage?
The movie increasingly plays on that ambiguity and on Martha's distrust and paranoia, teasing us with hints of resolution that it ultimately refuses to provide; the last half-hour or so, and the final scene in particular, are a frustrating exercise where we're never quite sure what Martha's seeing, or if she really is seeing what she thinks she's seeing.
Despite my irritation with the ending, I'd recommend the movie, because the performances are quite good. Olsen (who is, as I believe I am obliged to mention at some point, the younger sister of Mary Kate and Ashley) creates a very convincing portrait of a woman on the verge of complete emotional collapse, and in the earliest farm scenes, gives us a strong enough glimpse of who Martha once was that we can easily see the extent of her deterioration. Paulson is nearly as good as Lucy, who wants desperately to help but finds it nearly impossible to understand what the problem actually is; Hawkes oozes creepy charm and menace.