The true story behind the film "The Strangers" is an incident from writer and director Bryan Bertino's childhood, when someone knocked on the door of Bertino's home while his parents were away. His little sister answered, and the person asked for someone who didn't live at the house. Bertino later learned that people were knocking on doors in his neighborhood and breaking into the house if no one answered.
The incident from Bertino's childhood is the only true story related to "The Strangers." Bertino also received inspiration from "Helter Skelter," which he read as a child. The book is about the Manson family and their victims. Bertino specifically mentioned that he was interested in the victims and how they didn't know the Manson family or realize why they were targets.
Although the film is inspired by true events, the characters and events depicted in the film aren't real. The closet similarities between Bertino's incident and the film are the strangers coming to the door and asking for someone who doesn't live at the house. The strangers in the film have the opposite motivation of the strangers in Bertino's incident, as they state at the end of the movie that they were looking for whoever was home so they could murder them, as opposed to looking for empty houses to rob.