The moment Spencer meets Hope the summer before seventh grade, it's something at first sight. The pair become fast friends, climbing trees and planning world travels. After years of being outshone by his older brother and teased because of his Tourette syndrome, Spencer finally feels like he belongs. But as Hope and Spencer get older and life gets messier, the clear label of "friend" gets messier, too. Through sibling feuds and family tragedies, new relationships and broken hearts, the two grow together and apart, and Spencer, an aspiring scientist, tries to map it all out using his trusty system of taxonomy. He wants to identify and classify their relationship, but in the end, he finds that life doesn't always fit into easy-to-manage boxes, and it's this messy complexity that makes life so rich and beautiful.