Mystery in the Moonlight is a mystery that can help readers learn about a condition called XP, Xeroderma Pigmentosum. In the story, Nico and Isabel wonder if their new neighbors could be vampires or werewolves, since no one sees them in the daylight. They investigate and discover a different reason for their neighbors' living situation: Andrew, a child in the household, has XP and cannot be exposed to sunlight. When they make this discovery, Nico and Isabel find ways to include Andrew in moonlight activities with other neighborhood friends.