You remember the spooky parlor game Bloody Mary, a staple of slumber parties and Halloween gatherings. You stand in front of a mirror and say "Bloody Mary" over and over and wait. Soon a spectral image of a woman will appear, beside you, behind you, or maybe even taking over your own visage! The exact rules for the ritual of summoning Bloody Mary vary from place to place and over time. But who is Bloody Mary, anyway? What really happens when you do this? It could be anything, but you are very liable to scare yourself silly. PBS's Monstrum looks at the customs surrounding this game, the legends behind it, and the psychological factors that go into it. Your experience, imagination, and expectations have a lot to do with what results you get.