Simon Spier is a high school student with a loving family and close-knit group of friends. However, he harbors a secret: he's gay and hasn't come out to anyone, feeling uncertain about how his friends, family, and classmates might react. Simon's life takes a complex turn when he discovers an anonymous confession from a fellow student under the pseudonym "Blue" on a local message board, revealing that he is also in the closet. Piqued by their shared circumstance, Simon begins an email correspondence with Blue, and the two form a deep connection, providing each other with support and understanding, even though Simon is frustrated by his inability to identify Blue’s true identity.

As Simon navigates the typical trials and tribulations of high school life, his secret is put at risk after a classmate, Martin, stumbles upon Simon's email exchange with Blue. Martin, infatuated with Simon's friend, Abby, manipulates Simon by threatening to expose his secret unless Simon helps him win Abby's affection. Torn between his desire for privacy and fear of being outed, Simon tries to manage Martin's demands while keeping his friendships intact and pondering how coming out will affect those relationships. Amidst the tumult, Simon continues to communicate with Blue, whose identity increasingly dominates his thoughts as Simon becomes ever more curious, invested, and eager to meet the person behind the emails.