A troubled man starts working at a retirement home and realizes its residents and caretakers harbor sinister secrets. As he investigates the building and its forbidden fourth floor, he starts to uncover connections to his own past and upbringing as a foster child.

Two years after choosing not to kill the man who killed his son, former police sergeant Leo Barnes has become head of security for Senator Charlene Roan, the front runner in the next Presidential election due to her vow to eliminate the Purge. On the night of what should be the final Purge, a betrayal from within the government forces Barnes and Roan out onto the street where they must fight to survive the night.

Given the country's overcrowded prisons, the U.S. government begins to allow 12-hour periods of time in which all illegal activity is legal. During one of these free-for-alls, a family must protect themselves from a home invasion.

One night per year, the government sanctions a 12-hour period in which citizens can commit any crime they wish -- including murder -- without fear of punishment or imprisonment. Leo, a sergeant who lost his son, plans a vigilante mission of revenge during the mayhem. However, instead of a death-dealing avenger, he becomes the unexpected protector of four innocent strangers who desperately need his help if they are to survive the night.