Hannah is a not-so-normal kid who doesn't like looking at her face in the mirror. One day while she's having a sleepover with some of her friends, they tell her that they want to play a game called "Scary Mary". When she says she doesn't know how to play, they tell her the story: there once was a girl named Mary, and she was obsessed with her looks. One day, her farmhouse caught on fire and she burned to death. Today, she is waiting for someone to chant her name, so she could take their face...", and become beautiful. Wow!
To summon Scary Mary, you must hold a candle in front of a mirror in a dark room, and say a poem. Then turn on the lights and look in the mirror at the same time. Hannah carries out the task, but nothing happens yet. Days later Hannah finds a compact hairbrush with an M on it.
and drag and with them to an annual school event in the woods known as a Scary Mary party. It is apparently to honor a girl named who supposedly died in those woods.