In a dystopian near-future America, the government has sanctioned an annual 12-hour period in which all criminal activity, including murder, becomes legal. Known as "the Purge," this night is intended to serve as an outlet for aggression and a societal reset, supposedly reducing crime and unemployment rates for the rest of the year. On this night of sanctioned violence, emergency services are suspended, and citizens must fend for themselves. The affluent Sandin family, having profited from selling security systems designed to protect the wealthy from the chaos of the Purge, prepare to lock down their home and pass the night in safety. James, the father, assures his family that their state-of-the-art security will keep them safe, while his wife Mary struggles with the morality of the Purge.

As the night commences, the Sandins' sense of security is shaken when their son Charlie, showing sympathy for outsiders, disarms the security system to allow a bloodied stranger seeking refuge into their home. This act of compassion quickly spirals into a night of terror when a group of masked Purgers arrive at their doorstep, demanding the stranger be surrendered to them. The family is forced to make quick moral decisions, defend their home, and reevaluate their stance on the Purge as they fight to survive the horrors brought upon them and grapple with the consequences of their actions in this once-a-year lawlessness.