Sixteen-year-old Emma Briar feels like a freak when baby blue lines scrape down her shoulder blades. She refuses to look at the aching marks, but they won't be ignored. Slowly spreading, slowly darkening, they form an embarrassing tattoo that she struggles to hide. She despises them ... until meeting him. First, the weird gray lines carve into Conley Hughes's back. Then he shoots up several inches and puts on forty pounds of muscle - overnight. He's never felt more alone working out in public to explain away the added bulk. Out for a jog one day, the sight of a beautiful girl sprinting towards him freezes him to the spot. Her eyes are closed: the expression on her pained face, hauntingly familiar. It's like looking into a mirror. The unexpected surge of electricity that rattles their colliding bodies is anything but new to a secret onlooker. Something lurking in the shadows has hunted people with back lines for centuries. Conley and Emma will be hunted, too.