Watch VoirFilms. In the spring of 2002, filmmaker Joe Berlinger traveled to Vienna to witness the burial of the preserved brains of over 700 children killed at a Nazi "euthanasia" clinic. GRAY MATTER chronicles the filmmaker's personal journey as he searches for Dr. Heinrich Gross -- known as the "Austrian Dr. Mengele" -- who allegedly participated in these killings. Along the way Berlinger meets survivors of the clinic, as well as other remarkable individuals, who are confronting a nation that has only begun to grapple with its denial of this horrific legacy.