The film opens with Simon Spier (Nick Robinson) as he shares insights about his life. His family comprises his father, Jack (Josh Duhamel), a former quarterback, and his mother, Emily (Jennifer Garner), a valedictorian. Alongside him is his younger sister, Nora (Talitha Bateman), who dreams of becoming a chef. Simon considers himself pretty average, except for one significant secret: he is gay. One morning, he notices a handsome man operating a leaf blower and, despite his interest, his attempts at conversation turn out to be embarrassingly awkward.
During the school day, Simon rides to school with his close friends: Leah (Katherine Langford), Nick (Jorge Lendeborg Jr.), and Abby (Alexandra Shipp). The only openly gay student is Ethan (Clark Moore), who faces bullying from two obnoxious classmates, Aaron (Tyler Chase) and Spencer (Terayle Hill). Their principal, Mr. Worth (Tony Hale), humorously attempts to connect with the students while maintaining strict rules about cell phone usage.
Simon and Abby are set to perform in their school's production of Cabaret, much to the frustration of their teacher, Ms. Albright (Natasha Rothwell), who is tired of their lack of dedication. During a family TV session, an ill-timed joke from Jack about a contestant on The Bachelor regarding his appearance makes Simon cringe. Later, Leah informs Simon about a school blog featuring an anonymous student called "Blue," who is coming out as gay. Intrigued, Simon starts an email correspondence with Blue, adopting the alias "Jacques."
However, his secret is nearly exposed when classmate Martin (Logan Miller), known for his quirky behavior, finds Simon's emails and attempts to blackmail him into setting him up with Abby. Reluctantly, Simon agrees, trying to navigate the tricky waters of friendship and identity. During Halloween, Simon and his friends attend a party where he begins to suspect that Bram (Keiynan Lonsdale) might be Blue since they share a unique connection.
As Simon and his friends dress in creative costumes, Martin's flirtation with Abby goes awry as Simon unintentionally misleads Nick about her feelings. Amid the party’s chaos, Simon is crushed to find Bram involved with another girl, and things worsen when Martin embarrassingly vomits in front of everyone. Following the event, Simon spends a quiet moment with Leah, where she hints at her feelings for him, but he's too preoccupied with his own struggles to understand her.
Under Martin's pressure, Simon arranges rehearsals at the Waffle House with Abby and Martin, only to find that Martin is more focused on Abby than Simon's own dilemma. As Simon explores potential love interests among the waitstaff, he becomes increasingly frustrated with the dynamics of his relationships. The situation escalates when Martin publicly declares his feelings for Abby during the National Anthem, leading to significant embarrassment for all involved.
As winter break unfolds, Simon grapples with his own identity while attempting to come out to his parents on Christmas. They respond with support, yet Simon feels a gulf widening due to Jack's poor humor. Meanwhile, Martin's actions lead to Simon's emails being exposed online, resulting in a wave of alienation from his friends and classmates.
After a period of silence, Simon learns valuable lessons about acceptance and love from his relationships. In a poignant moment with Leah, he confesses his love for Blue, eventually coming to terms with his identity. After posting an open letter to the blog, he invites Blue to meet him following their school's performance of Cabaret.
The show goes off without a hitch, although an unfortunate letter mishap occurs during the performance. Simon waits anxiously for Blue at the carnival, surrounded by friends who have rallied around him through thick and thin. When Bram appears, Simon feels a rush of realization, and as they share a kiss, cheers erupt from their peers, transforming Simon's once solitary journey into a celebration of love, friendship, and acceptance.
In the end, Simon drives his friends to school, including Bram, with an optimistic spirit about life and relationships. They embrace the future together, ready for new adventures under the sunlit skies.
