Andrew Marantz traces how the unthinkable becomes reality: how alienated young people are easy prey, led down the rabbit hole of online radicalization; and how fringe ideas spread from festering in anonymous corners of the web, to being broadcast on cable TV.

What can we do to keep away “the gate crashers”—the white supremacists, conspiracists, and nihilist trolls who expertly use the internet to advance their poisonous agenda?