Long ago, seven demons tried to ursurp the Devil. As protagonist Maddie (Monica Keena) puts it, they were demons so badass that they wouldn't even follow Hell's rules. As punishment for their defiance, they were bound together and cast out of the underworld. They discovered a loophole, however, where if they could posess seven living people during the same Halloween night, they would again be free.
Eighty-four years prior to the events of the film, the demons last tried to accomplish this at the Broussard family mansion in New Orleans, but the seventh person, a woman named Evangeline, killed herself before she could be possessed. In the "present day" (2009) a woman named Angela Feld (Elizabeth) decides to throw a wild Halloween party at the mansion, in order to rake in some money. Among the party's guests are Suzanne (Luther) and Lily (Baird).Angela becomes infected through a bite from a skeleton found in the basement of the home, making her the first to be possessed. She seduces and then demonizes Suzanne, after having done the same to Lily's boyfriend, who also proceeds to demonize her. The demons are finally vanquished when Maddie, the only survivor from the party, manages to stall them until daylight arrives, which sends them back to the basement, perhaps lying in wait until another Halloween opportunity arrives.
The seventh demon is never revealed in the film, however, as each of the other demons only possess members of their own sex in both the 1925 and 2009 incidents, and the seventh targeted person both cases is a woman, it is likely that the seventh demon is female as well.