Simon Spier is a closeted gay high school boy living in a suburb of Atlanta, Georgia. He has a close and loving family—parents Emily and Jack, and sister Nora—as well as three best friends: Nick and Leah, whom he has known most of his life, and newcomer Abby.
One day, Leah informs Simon about an online confession of a closeted gay student at their high school, known only by the pseudonym “Blue”. Simon begins communicating with Blue via email using the pseudonym “Jacques”. The two confide personal details and form a connection. However, their emails are accidentally discovered by another student, Martin, who is infatuated with Abby. After learning his secret, Martin blackmails Simon by threatening to make his emails public unless he agrees to help Martin win over Abby. Simon begins to suspect that his classmate Bram is Blue.
At a Halloween party, Simon attempts to connect with Bram, but later walks in on him making out with a female student. During the party, Nick confides in Simon of his feelings for Abby. Simon lies to Nick, telling him that Abby has a boyfriend in college. Leah walks an inebriated Simon home, where she speaks vaguely about how she feels that she is fated to love one person very intensely; Simon believes she is referring to Nick.
Simon meets up with Abby and Martin at a local diner after he convinces them to practice lines together for an upcoming school musical. Simon bonds with their server, a classmate named Lyle, and believes that Lyle may be Blue. That night, Simon comes out to Abby and is relieved when she reacts positively.
At a school football game, Simon crosses paths with Lyle; before he can summon the courage to ask if Lyle is Blue, he finds out Lyle is actually interested in Abby. An upset Simon tells a pestering Martin to either “go big or go home” when courting Abby. Martin interrupts the national anthem and publicly declares his feelings for Abby. When Abby admits she does not share those feelings, Martin is humiliated and becomes the subject of intense ridicule.
On Christmas Eve, to distract people from his own humiliation, Martin outs Simon by posting his emails on the school’s gossip site. Simon’s sister, Nora, tries to comfort Simon but he shuts her out and does not return his friends’ frantic texts and calls. Simon comes out to his parents on Christmas morning, to their surprise but acceptance.
After the holidays, Nick and Abby, now a couple, angrily confront Simon about the lies he told and learn that he had tried to keep them apart due to Martin’s blackmail. Leah confesses to Simon that she was in love with him, not Nick, and is upset he came out to Abby first. After being rejected by his friends, Simon receives a final email from Blue, who is upset that their emails have been leaked. Blue tells Simon that they should stop speaking and deletes his email account. Simon is devastated, having lost his friends as well as the mystery pen-pal he has fallen in love with.
In the cafeteria, Simon and an openly gay student, Ethan, are mocked by two classmates. Ethan and Simon bond over the difficulties they have faced coming out. After his mother reaches out and comforts him, Simon apologizes to Leah. Simon posts a confession on the gossip site apologizing to his friends, seeking out Blue and asking him to meet at the school carnival.
After the school musical, Leah, Nick and Abby make amends with Simon and invite him to go to the carnival with them. Waiting for Blue at the carnival, Simon rides the Ferris wheel, drawing a large crowd of peers. When Simon runs out of tickets, Martin, making amends for his behavior, buys him one more ride. Just before the ride begins, Bram sits next to Simon, revealing himself as Blue; the kiss Simon saw with the female student was a drunken misunderstanding. They ride the Ferris wheel together and kiss as their friends cheer them on.
Simon’s life gradually returns to normal and he begins a relationship with Bram. While picking up his friends and boyfriend for school, Simon suggests that they forgo their usual morning routine and instead go “on a little adventure”.