Tilly Twomley is desperate for change. White-knuckling her way through high school with flawed executive functioning has left her burnt out and ready to start fresh. Working as an intern for her perfect older sister's start up isn't exactly how Tilly wants to spend her summer, but the required travel around Europe promises a much-needed change of scenery as she plans for her future. The problem is, Tilly has no idea what she wants.

Oliver Clark knows exactly what he wants. His autism has often made it hard for him to form relationships with others, but his love of color theory and design allows him to feel deeply connected to the world around him. Plus, he has everything he needs: a best friend that gets him, placement into a prestigious design program, and a summer internship to build his resume. Everything is going as planned. That is, of course, until he suffers through the most disastrous international flight of his life, all turmoil stemming from lively and exasperating Tilly. Oliver is forced to spend the summer with a girl that couldn't be more his opposite--feeling things for her he can't quite name--and starts to wonder if maybe he doesn't have everything figured out after all.

As the duo's neurodiverse connection grows, they learn that some of the best parts of life can't be planned, and are forced to figure out what that means as their disastrously wonderful summer comes to an end.