Your front door key is not unique. If a homicidal maniac walked from house to house for long enough, trying the same key in each front door, he might eventually open yours. Six-year-old Emilee finds this out the hard way when a smiling stranger coolly unlocks her suburban front door. He walks in and brutally attacks her family, then seems to vanish into thin air. Fourteen years later, the traumatic event haunts both Emilee and Gabriella, the homicide detective who handled the case. Together, they discover that similar home invasions have happened across America for decades. And when they launch a hunt for the 'key killer, ' they start to wonder if unnatural forces could be at work.