The movie opens with Charlie (Logan Lerman) writing a letter to an anonymous pen pal, discussing his upcoming first day in high school and his plan to turn things around by visualizing the last day of his senior year which is 1,385 days from that day. On his first day, Charlie thought that her sister Candace (Nina Dobrev) and her boyfriend Derek (Nicholas Braun) along with the Earth Club members would let him eat with them at lunch time, but they didn't. So he thought that his old friend Susan in middle school would want to have lunch with him, but Susan doesn't want to be her friend anymore. And then, Brad Hays which is his brother’s friend, but he thought that Brad is a senior and he's just a freshmen. On his shop class, one senior student decided to make fun of the teacher instead of the freshmen. On his Advance English class, he is harassed by his classmates but befriends his English teacher, Mr. Anderson (Paul Rudd).
Charlie attends a school football game alone and notices Patrick (Ezra Miller), who is the only senior in his freshman shop class. Patrick asks Charlie to sit with him. They are joined by Patrick's stepsister Sam (Emma Watson), and Charlie immediately notices her beauty. After the football game, they take him to King’s restaurant to eat dinner. Upon arriving home, he witnesses Candace's boyfriend hitting her, something Charlie is especially sensitive about as his Aunt Helen was also abused. Candace persuades him not to tell their parents.
Later on, Charlie gains enough courage and dances with Sam and Patrick at homecoming and goes to a party with them afterwards. There, he meets their other friends, Bob (Adam Hagenbuch), Mary Elizabeth (Mae Whitman), and Alice (Erin Wilhelmi). He also sees Sam with her boyfriend Craig, which makes him jealous. He eats a pot brownie and becomes high. The group is amused by his observations. While in the kitchen, Charlie admits to Sam that his best friend committed suicide the year before. He...