Born just 10 months apart, July and September are thick as thieves, never needing anyone but each other. Now, following a case of school bullying, the teens have moved away with their single mother to a long-abandoned family home near the shore. In their new, isolated life, July finds that the deep bond she has always shared with September is shifting in ways she cannot entirely understand. A creeping sense of dread and unease descends inside the house. Meanwhile, outside, the sisters push boundaries of behavior --- until a series of shocking encounters tests the limits of their shared experience, and forces shocking revelations about the girls’ past and future.