When her friend is picked on, thirteen-year-old Marin thinks nothing of standing up to a bully who outweighs her by fifty pounds. Her twin brother Josh has ADHD and can’t resist a dare. During a family beach outing, Josh is goaded into racing against one of his swim team members. But when a riptide ensnares him and starts to sweep him out to sea, he panics and begins to drown. In a frantic effort to help her brother, Marin dives under, but a dolphin saves him instead! With the help of the internet, the twins guess that their new ability is based on the Celtic legend of the Púca. At first it only manifests itself during severe emotional distress. The exhilaration they feel while using it is irresistible and almost intoxicating. Unfortunately, Josh’s control doesn’t come as easily as Marin’s does. And when a friend’s autistic brother goes missing, they come up with a plan to help.