Dateline: On Assignment

The notorious hacker, Marcel Lehel Lazar (also known as ``Guccifer''), who discovered Hillary Clinton's private email account; giant chimpanzees threatened by poachers; new insight into recovery from traumatic brain injuries.

Wildlife refuge Palmyra Atoll demonstrates what can happen when harmful things are removed from an environment; Americans who joined ISIS; Olympic swimmer Michael Phelps.

The Face of Terror - In an exclusive and unprecedented interview with Richard Engel, a New York City man who left the United States to join ISIS and then fled the group after five months is speaking out and revealing the disturbing details of his journey into the heartland of terror — and warning others not to follow in his footsteps.

The Clone Zone - Harry Smith and the 'On Assignment' team headed to South Korea, to the only lab that commercially clones dogs. The SOOAM Biotech Research Foundation in Seoul has already cloned 780 dogs for clients across the world. The price tag for a genetic duplicate of a puppy? Roughly $100,000.

Cynthia McFadden gets a rare behind-the-scenes look at Althorp, the childhood home of Princess Diana. The Spencer family is opening the doors to the 500 year-old mansion that features a history of noble guests and a world-class art collection among 100,000 square feet in order to raise funds for Whole Child International, the charity started by Countess Karen Spencer, American wife of Diana's brother, Earl Charles Spencer.

Lester Holt speaks to Airbnb co-founder and CEO Brian Chesky in Cuba about the revolutionary home-sharing website and app. Chesky will address criticism that Airbnb faces in cities across the world. Harry Smith explores the driverless car revolution with the godfather of the technology, and America's best-known car enthusiast, Jay Leno. Josh Mankiewicz reports on a ground-breaking approach to treating the nation's heroin epidemic.

TV host David Letterman; efforts to resolve distrust between cops and poor urban communities in Los Angeles; Steven Francesco, whose son died from a side effect of taking antipsychotics.