Twenty years ago, a young American hiker named Chris McCandless, the accomplished son of successful middle class parents, was found dead in an abandoned bus in the Alaskan wilderness and became the subject of the best-selling book and movie “Into the Wild.” Now, PBS retraces Chris McCandless’ steps to try to piece together why he severed all ties with his past, burnt or gave away all his money, changed his name and headed into the Denali Wilderness. McCandless’ own letters, released for the first time, as well as new and surprising interviews, probe the mystery that still lies at the heart of a story that has become part of the American literary canon and compels so many to this day.