Private McKinley Nolan vanished 40 years ago in Vietnam on the Cambodian frontier. Some say he was captured, some say he was a traitor, some even say he was an American operative. The US Army officially claims he was radicalized and went native, joining the Viet Cong and later encountering the Khmer Rouge. In 2006, retired US Army Lt. Dan Smith, revisiting the battlefields of his youth, may have encountered the elusive McKinley, alive. So began a journey into the heart of darkness.

• Documentary