Ostensibly an exploration of the damage a small, controlling cult can wreck on the individual, but I would make the case that it is equally and importantly a parallel exploration (intentionally or unintentionally) of mainstream social strictures. Brilliant!
While it is certainly a good movie, the only character that stands out is Martha. Her sister and the husband are more standard representatives of American modern society than they are real people. The man works, the woman is at home doing chores and sometimes friends come over for a party. So while the story of a girl adapting to society worked, I would've liked the sister and the husband being humans as well.
The comparison here between upper-class suburban life to a cult may be an interesting one, as both groups are about assimilation and following what you are told, unless of course you don't fit in, try to break free, or are not accepted. While the acting is good, the characters feel less like people, and more like a plot device for that message. Ultimately, the narrative is more repetitive and gimmicky than weighty.