Like most sixteen-year-old girls, Cassie O'Connell wonders what it's like to be in love. For Cassie, that question is even more daunting as ten years ago a virus wiped out every male on the planet. Well, almost every male. Cassie discovers a teenage boy, Jamie, who has been locked away in a secret isolation chamber at the lab where Amanda, Cassie's mother, works. Amanda searches for the cure to the virus-for the last ten years, she has kept Jamie hidden, to save his life and to use his blood to find a cure for the menace that still picks off male babies like a sniper. Then Cassie finds Sam, half dead in the mall parking lot. She helps him to recover from the bullet wound that almost takes his life, then joins him in his search for the murderous "man" who is responsible for a heartbreaking tragedy. Yet Cassie soon realizes that the mysterious boy is not what he seems. Will she survive Sam's thirst for vengeance and his need to know why he is immune? Or will she become a casualty of Sam's relentless quest? Cassie's growing feelings for the strange boy propel both her and Sam toward a destiny that will change them-and the world-forever.