The warning
(Book)

"Like most high school seniors, Alexandra Lucas is caught between living in the moment and an unknown future. Her anxiety disorder doesn't make that any easier. But she's coping—until her train stops on the way home from a concert with her boyfriend. At first, she's worried about breaking curfew. Then terror echoes through their train car. A mysterious doorway has appeared beside the tracks, and a hologram claiming to be a human from the future shares a sinister warning. A comet is on a collision course with Earth. All life there will end in six months' time. To survive, people must step through one of the many portals that have opened around the world. The holograms claim to offer safety. But how can anyone be sure? Stay or go--everyone must make their own choice. Alex's family, her friends, her boyfriend all have different ideas. Alex is only sure of one thing: she wants to decide for herself. But every decision comes at a price"