Clover and Danny are the kind of best friends who make each other better: Clover makes lists, and Danny makes fun. Clover is thoughtful and quiet and loves science. Danny is chatty and funny and loves art. They're so important to each other that Clover believes they're symbiotic: her favorite science word, which describes two beings who can't function without the other. But then Danny comes down with a mysterious illness that won't go away, and the doctors can't figure out what's wrong with him. So Clover decides to take matters into her own hands by making lists -- lists of Danny's symptoms, his good days, his bad days, and his moods. As the evidence piles up, only one thing becomes clear: Danny is only better when Clover is around. When they're separated, he's in danger. Clover knows they need to find a cure -- together. Will science be able to save Danny, or is this the one time when magic can overcome the unthinkable?