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March 1997. Thirty-nine members of the Heaven’s Gate cult are found dead in a Southern California mansion - the largest mass suicide in U.S. history. The inception of this mysterious group dates back to 1970s Oregon, where founding members Bonnie Nettles and Marshall Applewhite begin to recruit individuals interested in end-of-the-world Christianity, new age religion, and UFOs - but the disappearance of new members soon drives media attention and family concerns.

As the Heaven’s Gate "Classroom" begins, Bonnie and Marshall - who call themselves "Ti" and "Do" or "The Two" - enforce strict rules of no family communication. After Ti dies from cancer in 1985, a distraught Do briefly allows members to visit their families, and a few consider leaving the group for good. When the members return, Do becomes much more controlling - and opens the door to group suicide.

Using a popular sci-fi TV series to help bolster his teachings, Do encourages asexuality and individual submission for the benefit of the collective - and even initiates voluntary castrations. Having dwindled in number, the group starts recruiting again, but they’re met largely with derision and skepticism. Seeing this as a sign, Do begins planning their exit to the next level.

The 1997 arrival of the Hale-Bopp comet spurs the group to prepare for their departure, though their plan to commit mass suicide goes undetected. After an anonymous 911 caller alerts the authorities, Heaven’s Gate, deemed the first "internet cult," becomes a media and pop culture phenomenon - but the suicides have a very different effect on those closest to them. Today, former members, loved ones, and experts reflect on the legacy of Heaven’s Gate, offering a chilling reminder that indoctrination can happen to anyone.