The sensitive target of vicious bullying at school, 12-year-old Owen (Kodi Smit-McPhee) is a social misfit from a broken home. By day Owen dreams about laying waste to his classroom tormentors. One evening, as Owen takes out his pent-up aggressions on a tree, his new neighbor Abby (Chloë Moretz) appears over his shoulder. A young girl wise beyond her years, Abby just moved in next door to Owen with her stoic caretaker (Richard Jenkins) who seems to harbor a sinister secret.
Owen senses a kindred soul, and strikes up a friendship with the girl, despite her repeated attempts to maintain an emotional distance. Simultaneously, their community grows vigilant following a series of vicious murders, and Abby's caretaker vanishes without a trace. As Abby begins to grow vulnerable, her bond with Owen strengthens. By the time Owen begins to suspect that his evasive new friend is something other than human, it starts to seem as if Abby could use a good friend after all. Given that his bullies are growing more emboldened by the day, so too could Owen.