Colin Singleton is a former child prodigy who is obsessed with math and relationships. He has been dumped by 19 girls, all named Katherine, which leaves him feeling insecure and uncertain about his future.
After his latest breakup, Colin goes on a road trip with his best friend Hassan. They end up in a small town in Tennessee, where they meet Lindsey and her family. Colin spends the summer there while working on a mathematical “Theorem” that he believes can predict how relationships will begin, last, and end.
As the story progresses, Colin gradually realizes that his theory cannot truly explain or control love and life. He begins to move past his obsession with the Katherines and learns to accept uncertainty.