In a global cataclysm, “The Sudden Departure,” 140 million people disappeared without a trace. Three years later, residents of Mapleton, N.Y., try to maintain equilibrium when the notion of “normal” no longer applies. Intense grief has divided families and turned faith to cynicism, paranoia, and madness, leading some of the traumatized to join the Guilty Remnant, a cultlike group. Kevin Garvey, a beleaguered police chief, must keep the peace between townspeople and the cult, a task made tougher with concern about his kids. His daughter alternates between apathy and rebellion, and his wayward son befriends a charismatic prophet. “The Leftovers” is based on the best-seller by Tom Perrotta, who is one of the series’ executive producers.