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.

After an Army hearing frees Green Beret Dr. Jeffrey MacDonald, his father-in-law demands a reinvestigation. But when he sees all the evidence, he's shocked by where it leads.

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.

Jeffrey MacDonald served more than 30 years in prison when a federal judge finally agrees to hear the new evidence in his case. Errol Morris heads to North Carolina believing the truth will finally come out.