In 1991, Charlie, who has suffered from clinical depression since childhood, has been discharged from a mental health care institution. Uneasy about beginning his freshman year of high school, he is shy and only manages to make friends with his English teacher.
Charlie meets two seniors, Patrick and his stepsister Sam. After the homecoming dance, Sam and Patrick invite him to a party with their friend group. He unknowingly eats a cannabis brownie, gets high, and makes speeches to the amusement of the party-goers. He also walks in on Patrick and Brad, the high school quarterback, kissing. Patrick tells Charlie that Brad is closeted, so he agrees to keep it a secret. Charlie discloses to Sam that his best friend committed suicide the year before, making her realize that Charlie has no other friends, so she and Patrick bring Charlie into their group. On their way home, the three hear an unknown song on the radio. Sam instructs Patrick to drive through a tunnel so she can stand up in the back of the pickup while the music blasts.
Sam needs to improve her SAT scores to have a better chance of being accepted into Penn State University, so Charlie offers to tutor her. This improves her scores, and at Christmas, she gives him a vintage typewriter. They discuss relationships, and Charlie reveals he has never been kissed. Sam reveals that her first kiss was at age 11 by her father's boss. Charlie reveals that his Aunt Helen was also sexually assaulted as a child. Sam tells Charlie she wants his first kiss to be from someone who loves him, and they kiss.
At a party, Charlie trips on LSD. He cannot control his flashbacks of Aunt Helen, who died in a car crash on her way to buy him his 7th birthday gift. He ends up in the hospital after falling asleep in the snow. At a regular The Rocky Horror Picture Show performance, Sam asks Charlie to fill in for her boyfriend, Craig. Their friend Mary Elizabeth is impressed and asks him to the Sadie Hawkins dance, and they enter an unsatisfactory relationship. At a party, when Charlie is dared to kiss the prettiest girl in the room, he chooses Sam, upsetting both her and Mary Elizabeth. Patrick tells Charlie to stay away from the group for a while and Charlie sinks into depression.
Brad shows up to school with bruises on his face after his father catches him having sex with Patrick. Brad says he was jumped and beaten up, and he distances himself from Patrick, calling him a "faggot". In anger, Patrick punches him, causing him to retaliate. Brad's friends beat Patrick and prevent Sam from intervening when Charlie does before blacking out. Upon recovering, he finds that he has incapacitated Brad's friends. Sam and Patrick thank Charlie, and the three become friends again.
Charlie's mental state worsens. Patrick tries to cope with what happened with Brad and kisses Charlie before immediately apologizing. Sam is accepted into Penn State, and she breaks up with Craig on prom night after learning he is cheating on her. The night before she departs, she brings Charlie to her room. They confide in each other and kiss, but when Sam touches Charlie's thigh, he experiences a flashback of his Aunt Helen, which he passes off.
After Sam leaves for college, Charlie's emotional state and flashbacks worsen. He calls his sister, who realizes he is in distress and calls the police. Charlie eyes a kitchen knife as they burst through the door and awakens in a hospital, where psychiatrist Dr. Burton brings out his repressed memories, revealing that his aunt sexually abused him as a child.
The night Charlie is released from the hospital, he is visited by Sam and Patrick. The three revisit the tunnel, having identified the song (Heroes by David Bowie), and Charlie and Sam share a kiss. Charlie stands up in the back of the truck. He acknowledges that he feels alive and that, at the moment, "we are infinite".
