To eliminate the non-contributors in GDP of the national economy, the state of America has the day called The Purge. In the state fully-fleshed with crime, criminals and prisons, government sanctions annual purge, a period of 12 hours, during which commiting any crime is legal. One can kill people, harm them or torcher or so whatever they want with them, but in just the period of 12 hours.
Police won't help anyone, so there is no sense of calling them. Hospitals temporarily down for help. The only night on which committing crime isn't really a crime. The thought behind The Purge is to remove those who are destroying the country like the beggars, people sleeping on footpaths, or those who are helpless, can not protect themselves.
The family is here the main characters, and son of the couple helps a man, who doesn't finds help from anywhere. He opens the doors and lets the stranger in. The people executing purge outside threaten Sandins and advice them to give the man back. Otherwise the purge would be unhappy for their whole family.
Same night, Zoey's boyfriend who is in the house is killed by Mr. Sandin unintentionally. Later, when Mr. Sandin is ready to hand over the man to the people outside, other members in the house are really disappointed as he is going to get a man killed to save themselves.
He then decides to change his decision. Intruders start violence at the house. The doors broken and they are in. Mr. Sandin tries to save the family and he is successful, until his neighbours arrive for help. At the same time, Mr. Sandin dies. The neighbours are actually not there to save them, instead to execute a purge on the family to take a revenge of being looted by Sandins.
The stranger helps Mary to get a control over the situation and makes all of the remaining people sit till it's 7:00 a.m. Meanwhile, when the lady tries to attack Mary, Mary counter-attacks her to put her nose broken, bleeding badly.