Krista is convinced her sister, Sadie, is possessed by a demon. It's the only plausible explanation for how cold and distant she's become since reaching high school. And it's why Krista bought the strange book online: to destroy the demon and get her sister back. But when their summoning ritual goes terribly right, Krista and Sadie realize too late that what they brought into the world hasn't come to help. Trapped in their apartment building with a horde of bloodthirsty monsters, the sisters need to fight to not only exorcise the demons slaughtering their friends and neighbors, but also the ones that drove the two of them apart.