Simon Spier, a high school senior, leads and has led what he considers a perfectly normal perfect life. He has a loving family in his parents Emily and Jack Spier, and his younger sister Nora Spier. He has a close group of friends in Leah, Nick and Abby, the former two who he's known almost all his life, and the latter who just transferred to their school, Creekwood High, this year, but who they feel like they've known just as long. But there is a "but" in his life, the big secret that he's been keeping now for a few years: he's gay and in the closet to everyone. He knows especially his parents would be all right with his homosexual orientation, despite Jack's often politically incorrect jokes, and that most of the students accept Ethan, their one and only openly gay classmate, but Simon has failed to come out for various reasons, mostly having to do with the change that would occur in what he has known as his life. He discovers that he is not alone in another classmate calling himself "Blue" (a pseudonym) posting online his own struggles in being gay and in the closet. With that posting having an email address where one can respond, Simon begins an anonymous correspondence with Blue opening up about his own struggles. In the support they end up providing each other, Simon begins to fall for Blue not even knowing who he is. As Simon tries to figure out who Blue is, he starts to see his classmates in a new light. But circumstances lead to Simon manipulating his friends' love lives all in an effort to maintain his secret.