Famed filmmaker Errol Morris is obsessed with one of America's most popularized true crimes: the murder of a pregnant wife and two young daughters on an Army base in 1970. Green Beret doctor Jeffrey MacDonald said a band of drugged-crazed hippies attacked his family, led by a girl in a floppy hat. Army investigators said MacDonald committed the murders.

Nine years after the murders of his pregnant wife and two young daughters, the guilt of Green Beret Dr. Jeffrey MacDonald is put to a civilian jury. The defense calls a surprise witness: The Girl in a Floppy Hat.