​Eleven year old Rosie has always been able to sense colors — and she thought everyone else could too. After the first meeting of the middle school math league ends in confusion, Rosie wonders why she’s suddenly the only one who knows that every letter, every number, and every song has a special hue of their own. Things only get more complicated for Rosie when the only thing her best friend Harper seems to care about is impressing her new, cooler friend who insists there’s no such thing as “brain colors.” Desperate for someone who understands, Rosie befriends Yara, the oddball music geek who spends all her time in the school band room. Things finally start making sense when Yara tells Rosie that her condition has a name—synesthesia.