Book excerpt: Like the yearning, doomed young clones in Ishiguro's Never Let Me Go, three teenagers with XP (a life-threatening allergy to sunlight) are a species unto themselves. As seen through the eyes of 16-year-old Allie Kim, they roam the silent streets, looking for adventure, while others sleep. When Allie's best friend introduces the trio to Parkour, the stunt-sport of running and climbing off forest cliffs and tall buildings (risky in daylight and potentially deadly by darkness), they feel truly alive, equal to the "daytimers." On a random summer night, while scaling a building like any other, the three happen to peer into an empty apartment and glimpse an older man with what looks like a dead girl. A game of cat-and-mouse ensues that escalates through the underground world of hospital confinement, off-the-grid sports, and forbidden love. Allie, who can never see the light of day, discovers she's the lone key to stopping a human monster.