Clover and Danny are the kind of best friends who make each other even better. 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 when Danny comes down with a mysterious illness that won t go away, the doctors can t figure out what s wrong with him. So Clover decides to take matters into her own hands by making lists list of Danny s symptoms, his good days, his bad days.