Colin finds Lindsey at her secret hideout in a cave, where he tells her the story of every Katherine he has ever loved. Lindsey tells him how she does not feel sad but instead slightly relieved by Lyford's affair. They discuss what it means to them to "matter" and eventually confess their love for each other. As their relationship continues, Colin decides to use his dating formula to determine whether he and Lindsey will last. The graph reveals that they will only last for four more days. Lindsey slips a note under his door, four days later, stating that she cannot be his girlfriend because she is in love with Hassan. But she leaves a P.S. stating that she is joking. Colin realizes that his theorem cannot predict the future of a relationship; it can only shed light on why a relationship failed. Despite this, Colin is content with not "mattering". Hassan says that he is applying for two college classes, which Colin has been trying to convince him to do throughout the book. The story ends with the trio driving past the restaurant they were originally planning to go to, because Colin, Lindsey, and Hassan realize that they can just keep driving; there is nothing stopping them from continuing on.